There are so many for internal server error. The main reasons for internal server errors are.
- Corrupt .htaccess file
- PHP Memory limit
- Corrupted plugin
- Incompatible PHP version
- Corrupted core files
In oder to fix this issue, you need to investigate in step by step order.
To solve this issue first of all you need to enable Debug mode and check the issue. after that try with restoring .htacess file, enabling default theme, disabling plugins etc.
if you still facing 500 internal server issue, check the tutorial on wpera –> https://www.wpera.net/500-internal-server-error/