Joomla website returns 500 error

I've ran into a problem which I'm unsure how to approach. I have a Joomla based website (that I didn't code but rather maintain) that went offline 2 days ago. Trying to fetch the website the server retus a 500 error. Looking around Google first advise was to reinstall joomla's theme and extensions, but the administration panel for Joomla is also down, all the pages retu a 500 error. Also when I download some file through ftp I get a trojan waing on "index.php" and "error.php". What puzzles me is first that I can't get to any page and I'm wondering if that could at all be a problem with the website's files or it must be a problem in the server's OS. Secondly is there any way for a trojan to get into my (personal) system from a .php file, especially if I tu off my localhost I don't think an execution of the file should be possible. PP: the database of the website seems untouched

I'll be happy to receive any suggestion on how to tackle that, it's just that I'm stuck.