Help! iMac displaying startup screen with code

It looks like your iMac is booting into verbose mode.

Start up your Mac in single-user mode or verbose mode - Apple Support

From the terminal application you can enter the following to turn off verbose mode:

sudo nvram boot-args=""

Power it down, then restart. If that doesn't help, restart with the Shift key down to put it in Safe Mode.

Use safe mode to isolate issues with your Mac - Apple Support

Anything changed recently? Without any real information, my first guess would be bad or mismatched RAM.