Oh, noooo. You got most popular developer error 🙂
Firstly, No despair – say “Yes, I CAN”, and concentrate on possible errors.

Show all errors (how to show all errors/warnings/nootices in php)
Also, you can do “fast debug” in main parts of you code (Sure, use die() command but very carefully)

Also, if you using POST method with uploading files then check php settings in php.ini (max_post_size, max_executing_time…)

Standard Apache logs dev finiding here:
/var/log/apache2/...
nginx logs here:
/var/log/nginx/...
Also, some popular frameworks logs folders:
Continue reading If you got 500 (Internal Server Error) PHP/Nginx/Apache – Where you can found logs/reason?