With the info you provided it’s difficult to say.
Error 500 happens because you did some error in the code that is supposed to produce the page, or the code generates some unhandled exception.
My suggestion is to visit the page that gives you the 500 error, and then try to comment out all your code. See if the issue is still present. If not, uncomment the code until you find the critical part that originates the error. Could be anything, a typo, a file not found, a logical error, anything.
Also, check in the webserver logs, if you can read them.