Sudden error for site, http 500?

Was away from the site for several months so I don't know exactly when it began, this error shows on any part of the archive save for html files I wrote myself, which are totally fine. I see mails from the host announcing ph 8.1 then 8.2 upgrades, could this have caused the issue? I tried backing up and replacing sql tables, folders for the site itself and nothing changed. Any ideas? I also plan to update to the newest release as soon as I am able.