Mac wont start after Big Sur

Thank you for your input. I tried this and several other online suggestions. Right now I have reinstalled Mohave. I tried a fresh update to Big Sur and everything went well until the very end where it says it has to shut down to complete the installation. It shuts down, won't complete install and becomes a brick. The only way I can get back in is Power/Command-R to disk utility. Now I running Mohave but can't turn off the computer at all. It won't restart. It wakes up fine from sleep mode.

Just a quick follow up and "Thank you". I went through the resets multiple times, but computer still wouldn't boot up (even in safe mode). Reinstalled Mohave and stayed with that until I could see a significant window of time. These failed upgrade attempts were leaving me stranded for hours. Tried the resets again. I also opted to install any Mohave patches or updates I could find. Not sure which one did the trick but when I hit the upgrade to Big Sur button, it worked! The process went smoothly, and the computer rebooted just like it should. Thank you again for following up and your suggestion.