Prohibited sign, external hard drive removed, now no disk

Does the iMac normally startup from the usual internal drive, Macintosh HD?

Or was it set up to boot from the external drive that is now disconnected?

Please look over this support document carefully.

How to reinstall macOS from macOS Recovery - Apple Support

1. Leave the external drive disconnected. Don't risk the photos at this point.
2. Reboot the Mac in Recovery using Command-R (⌘R).
3. Decide whether to erase (probably not necessary, but might be required.)
4. Choose Reinstall macOS. Select the internal disk drive to install on – this is usually Macintosh HD
5. Follow the prompts and allow the process to complete.

Prohibitory symbol

A circle with a line or slash through it means that the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. You should reinstall macOS on that disk. Try the following which will not erase the disk.

Reinstall El Capitan or Later Without Erasing Drive

Please be sure you back up, if possible.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the side list.
4. Click on the First Aid button in Disk Utility's toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Install OS X and click on the Continue button.

Please read Writing an effective Apple Support Communities question. We are here to help. We are all users. We are not mind-readers nor clairvoyants. We only know what you write down and tell us. Be specific and thorough. Include your hardware information and installed version of macOS.

OrangeBattery Said:

“Prohibited sign, external hard drive removed, now no dis:[...] Only thing I could find was to erase/format the disk, but it is full of photos, so my wife disconnected it. Now no disk is showing, and I’m at a loss as to what to do now. Is there a way to select a different disk, the ssd perhaps?”

———-

Try The Following:

A. Boot with this Plugged in:

1. Shut Down: the Mac
2. Unplug: the External Hard Drive for 30 Seconds
3. Bootup: the Mac
4. Shut Down: the Mac
5. Plug In: the External Hard Drive
6. Wait: 30 Seconds
7. Reboot: the Mac
8. Log In as the Administrator

Is it now seen?

If Not..

B. Mount the Disk Recovery Mode

1. Bootup: into Recovery Mode (Hold Down: [“command” + “r”] upon boot
2. Choose: Disk Utilities
3. Mount: the Disk
4. Restart: the Mac
5. Log In: as the Administrator

Is it now seen?

C. One More Thought:

I would also try a new administrator and see if the above works. If so, then it is something configured in your current user.