Lightroom 6.0 on MAC. Updated to MacOS Monterrey yesterday and now lightroom 6.0 won't open

I don't mind updating to creative cloud at this point

You should seriously consider that, especially when running Monterey. LR6 is unsupported and a bit wonky (non technical term) under that OS. You'll be asked to update your old catalog, which as others have stated, have all your edits and such, your images still reside wherever they were in the first place. The old catalog will remain as a back up. 

BUT have I just lost 5 years worth of photography projects and personal images??

No, all of your work is in your catalog file, which you should be able to access if this LR 6 problem can be fixed, or if you upgrade to Lightroom Classic. Your photos are still there too.

A possible fix is to rename or delete your preference file. https://www.lightroomqueen.com/how-do-i-reset-lightrooms-preferences/

Have a look at the info available at the link below and carry out the procedures.

If Lr 6 still fails to boot then go to the folder that contains the Lightroom Catalog which by default would be in your "users > Pictures> Lightroom" sub-folder. Open the Lightroom folder to display its contents as a list. You should see something like the screen capture below.

If there is a Lock file present delete that file and not any others. Do not delete the Catalog file.

After deleting the lock file double click on the Catalog file and this should boot Lightroom with your Catalog.

If there is more than one Catalog the info displayed could give a clue as to which one to use. The date last modified is useful.