500 Server error FIXED, What To Do Now!?!?!?

Hello, I was having a 500 server error issue, I tried multiple different things that didnt work, I then tried renaming “themes” folder in wp_content to “_themes” it worked my site came back up along with admin.

My problem now is, how do I find the problem?? I have been running this theme for over 3 months now with no issue and now when I go back a rename the file to original I get the sever error again.

And, before I this all happened, I was editing my functions.php file.
I added something to it but ended up taking it out just before I got the error message.

Thanks!!