My iMac shows a Lock Screen

That looks like a firmware password lock has been activated. If you don't know the password, then you will need to take the Mac to an Apple Store or an Apple Authorized Service Provider along with valid proof of purchase & ID so the firmware lock can be removed. Usually you only see this firmware password screen is when you Option Boot a Mac with a firmware password lock enabled. If you are not trying to Option Boot the Mac, then it either means the Shift key on your keyboard is stuck or the Mac's internal drive is not bootable most likely because the internal drive has failed.

Here is an Apple article with details on the firmware password lock:

Set a firmware password on your Mac - Apple Support

Please read and follow the advice in Avoiding image persistence on Apple LCD displays - Apple Support (AT)