Could be a lot of things. Not much to go on!
Some simple things would involve clearing local cache or trying from a different browser.
That said, the apache error logs would likely have some info for us in
Anything recent will be at the bottom of the file. Server 500 errors usually get logged as a Critical Error. If you can find any of these feel free to post them here, or a large chunk/all of your log if you want to throw it in a pastebin. That said, please take care to wipe any sensitive info from the log, or PM myself the link only.