External monitor not detected after sleep or wake

After this, the monitor comes back on its own every time the Mac wakes, no replugging required.

Why this happens

This is a display handshake problem, not a data-loss or system risk, which is why every fix below is at most "caution." Docks and hubs make it more likely because they add another handshake in the chain (Mac → hub → cable → display) and a bigger gap for one link to fail to renegotiate after sleep — Apple Silicon Macs power down the Thunderbolt/USB controller more aggressively on sleep than Intel Macs did, which is widely reported (though not Apple-documented) as the reason docks specifically get worse after this transition. It's a separate mechanism from something holding the Mac awake, but the two get reported together often enough that it's worth ruling that one out too if the display issue only shows up on Macs that also aren't sleeping properly.

One thing not to bother with: resetting NVRAM/PRAM. Apple's own support documentation states plainly that the NVRAM reset steps "don't apply to Mac computers with Apple silicon and aren't needed on those computers" — if an older guide tells you to reset it for a display problem, that guide is written for an Intel Mac.

Fix it

1. Ask macOS to redetect displays before touching cables: open System Settings → Displays, then hold Option — the "Detect Displays" button appears (it's hidden until Option is held). Click it. This alone resolves the handshake failure most of the time.

2. If that didn't bring it back, confirm what the GPU actually sees right now:

   ● Reads the system

   What it does: lists every display the GPU currently has a handshake with, and its negotiated resolution.

   After: nothing changes, this only reads.

   system_profiler SPDisplaysDataType

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   Output

   Displays:
       Color LCD:
         Display Type: Built-in Liquid Retina Display
         Resolution: 2560 x 1664 Retina
         Online: Yes
       PL2792Q:
         Resolution: 2560 x 1440 (QHD/WQHD - Wide Quad High Definition)
         UI Looks like: 2560 x 1440 @ 70.00Hz
         Online: Yes

   Captured live on the verification machine with an external monitor attached — both displays report Online: Yes. If your external display is missing from this list entirely, macOS isn't seeing it at the port level and the problem is upstream of software — go to the cable/hub check below. If it's listed but the screen is still blank, the handshake is stuck and step 3 is next.

3. Rule out the physical chain: try a different cable, a different port on the Mac, and if you're using a hub or dock, plug the display straight into the Mac instead. This is the single most common actual cause once redetect fails — a marginal cable or an underpowered hub that drops the link the moment the display goes to sleep.

4. If it's still stuck, macOS may be holding onto a stale cached display profile (EDID) for that monitor from before it went to sleep. Clearing the cache forces it to read the monitor's real capabilities fresh on next login:

   ▲ Changes settings

   What it does: deletes the cached per-display configuration for your user account.

   After: log out and back in (or restart); display arrangement and any per-monitor calibration you set are forgotten and rebuilt from scratch.

   rm ~/Library/Preferences/ByHost/com.apple.windowserver.*.plist

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   Revert: nothing to restore — the file regenerates automatically; you'll just need to re-arrange displays if you had a custom layout.

5. If the display shows as online in step 2 but stays blank and none of the above helped, restarting the window server forces every display handshake to redo from scratch. This logs your current session out immediately — save your work first.

   ▲ Changes running state

   What it does: force-restarts the display server (WindowServer), which redoes every display handshake.

   After: your GUI session ends immediately and you land back at the login screen — any unsaved work in open apps is at risk exactly as it would be from a forced logout.

   sudo killall WindowServer

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   Revert: nothing to restore — WindowServer relaunches on its own and you log back in normally.

Still broken?

• Mac won't go to sleep on its own — worth checking if the display issue only ever shows up alongside sleep problems.
• Mac fan running loud and stuck at max (thermal throttling) — an external display adds real GPU load, and on a base-chip Mac that's sometimes enough to tip it into throttling.

Verified on

The system_profiler output above was captured live on the M3 / macOS 26.6 row with a real external monitor attached. Before assuming a second monitor is a software bug, check whether it's actually a hardware limit — the native external-display count is per chip tier, not just per generation: base M1/M2 laptops (MacBook Air, non-Pro MacBook Pro) drive exactly one; the M1/M2 Mac mini can drive two despite the same chip, since the one-display cap is a laptop restriction, not a chip one; base M3 laptops still drive one with the lid open, but gained a second display in clamshell mode only (lid closed, external keyboard/mouse) via a macOS Sonoma 14.6 software update; base M4 is the first non-Pro chip to drive two displays with the lid open, no clamshell required. Pro and Max tiers across all generations support two to four displays depending on the exact model — check Apple's current support page for your specific machine before troubleshooting a "missing" monitor that was never going to appear.

Related problems

• Mac won't sleep even with the lid closed
• Mac fan running loud and stuck at max (thermal throttling)