With a 24 inch iMac M4 the first question is how far it gets, because that decides which fix applies. A desk machine has no battery to drain, so a completely dark screen with no sound usually means the power cable, the outlet or the power button, and it is worth reseating the magnetic power connector at the back and trying another outlet. If you hear the start-up sound or the screen lights and then stalls, the iMac is running and the fault is in software or in something plugged in.
The checks in more detail
Pull the cable from every USB-C port and the Ethernet port, and remove any hub, drive or card, since one faulty device can hold start-up at the logo. A white or gray screen with a progress bar that crawls is the iMac applying an update, so give it a few more minutes. The Magic Keyboard that came with it can also confuse a login: it connects over Bluetooth, and if the login window will not take your password, plug it in by its cable once and try again.
If it still stalls, open the startup options by shutting the iMac down and holding the power button, which is on the back edge at the left, until a window with a gear marked Options appears. Apple's page describes what the screens mean: a folder with a question mark says no startup disk was found, a prohibitory sign says the system is damaged or does not match, and a globe says it is looking for macOS over the internet.
Choose Options, then Disk Utility, and run First Aid on the internal volume; if it reports errors it cannot fix, back up through Share Disk from another Mac. Reinstall macOS leaves your files in place, and Time Machine can bring back an older state. Apple Configurator on another Mac with a USB-C cable can revive a Mac whose Options never appear. Book Apple Support if none of this works, since iMacs have no user-serviceable parts.
Differential
Likely causes of “Apple iMac 24-inch is stuck at startup, automatic repair or recovery”, and how to tell them apart
- 1
A stalled macOS update on the desktop
What separates it The iMac sits on the Apple logo
How to confirm Choose Options at start-up and reinstall macOS
- 2
A damaged internal volume
What separates it A question mark or prohibitory sign
How to confirm Open Disk Utility in Recovery and run First Aid
- 3
A USB hub or drive that hangs the boot
What separates it The iMac stops after the chime
How to confirm Unplug every USB device and start again
- 4
A keyboard that is not connected at login
What separates it The login screen cannot be used
How to confirm Plug in a wired keyboard
- 5
An Apple Account lock on a second-hand iMac
What separates it A restore asks for credentials
How to confirm Ask the seller to release it
Before you touch a setting
Record this first — changing settings destroys the evidence
- The symbol or logo on the 24-inch screen
- Whether a wired keyboard is connected
- What was plugged into the iMac's ports
- Whether Time Machine is current
Try in this order
0 of 6 completedShut the iMac down
ReversibleThen hold its power button until Options appears.
Unplug every USB device
ReversibleHubs, drives and dongles.
Run First Aid
ReversibleIn Disk Utility on the internal volume.
Reinstall macOS
ReversibleWithout erasing the disk.
Use a wired keyboard
ReversibleIf the login screen ignores the wireless one.
Restore from Time Machine
ReversibleIf the reinstall fails.
Model-specific
What is true of this model and not of its siblings
- Apple silicon
- Recovery is a power hold.
- Revive
- Another Mac can restore firmware.
Stop boundary
When this stops being a self-service repair
- !If Options never appears, the iMac needs an Apple revive or restore
- !If First Aid keeps reporting errors, back up and call Apple Support
- !A seller must remove their Apple Account before a restore
Evidence ledger
Sources behind this answer
Exact questions
Common questions
How do I open Recovery on an iMac M4?+
Hold the power button until Options appears.
Why is the login screen unusable?+
A wireless keyboard may not have connected.
Will a reinstall erase files?+
Not unless you erase the disk.
What if Options never appears?+
Apple needs to revive it.