I have nexus ultimate and have been running ok but today when I wake my computer I get a pop up with the heading Warning: software protection mechanism triggered....and some other info and nexus closes. I am running windows 11 and I have BitDefender antivirus. As I said it has been running fine up until today...any suggestions.

You don't give any detailed info such as version nos., where the warning is from (i.e., e.g., Nexus, Window, or BitDefender), full text of the warning notice, what happens if you then start up Nexus again, etc.

Given that BitDefender can cause all kinds of problems seemingly out of the blue, that may, or may not, be the cause of your problem....

Read this. I assume you didn't notice it. It's the very first sticky.
viewtopic.php?f=2&t=10604

Probably another case of, 'why bother with all that, let someone else give me the answer....' - I think in future I'll just ignore posts like that.