how to reboot my iMac using a PC keyboard. I have Montery OS and it crashes often

What do you mean reboot using a pc keyboard. You don’t even need the keyboard, only a mouse to reboot. Please explain in detail what you are to achieve.

I understand what you're saying. I have a PC keyboard on my Mac Mini. The answer you're searching for is the WIN and R keys. However you need to hold them both 'first' and then press the power button. I tried several times before it worked before I learned that...

Imacs (or any Macs) don't restart for any reason, that is what PC's do. Please do the following and we can look for obvious issues.:

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

Use the Apple Menu at the Upper left of the screen and select "Restart...", or "Shut Down..."

Also, try to start in Safe Mode and see if it crashes there:

https://support.apple.com/en-us/HT201262

You select Restart after clicking the Apple symbol in the upper left of the display.