A 500 error is a general server-side error. Common causes and fixes:

1. Bad .htaccess: a syntax error here is the most frequent cause. Rename .htaccess (article 105) to test; if the site returns, fix or regenerate the file (WordPress: re-save permalinks).
2. PHP memory limit: raise it (article 97) if the site ran out of memory.
3. Faulty plugin or theme (WordPress): disable plugins (article 96) to find the culprit.
4. Wrong file permissions: files should be 644, folders 755 (article 112) — never 777.
5. Check the error log: Metrics > Errors in cPanel usually shows the exact cause.

Tip: The error log is the fastest route to the real reason — start there.