The error above occurs when website is migrated to some other server from one server
Why is this problem
1) When website was created on a server, then its home url and site url are set to previous server domain.
2) There can be some htaccess poblem too but mostly it is first one.
1) Open you database and upate the wp_optins tabes and change the site url and home in wp_ptions table
There can be other issues like you have nt placed the website in right directory.
To check if htaccess is working , edit htaccess and just place some random string or character and then upate it.
If you Internal server error :500 , it means the htaccess is working.
You can continue debugging further.