A 500 error could literally be anything on the server, I can think of hundreds of different errors that will cause a web server to respond with a 500 error. Without more specific information there's no way anyone can help you other than to guess.. so here's my guess.

If you're getting a 500 error with a PHP script, try changing the permissions to 644 and see if that helps. Some scripts recommend setting permissions for certain files or directories to 777 but a lot of hosts don't require that (and aren't allowing it) these days for security reasons.

If you're getting a 500 error with a CGI script (something in your /cgi-bin/ or anything with a .cgi or .pl extension) try changing the permissions to 755 and see if that helps.

If it's not a permissions problem, nobody here can help you without more specific information (like the script that's giving you the error and the relevant lines from your server's error log), and even then it might be an issue that only your host can help you with (could be a mod_security filter, for example).

Thanks for your answers. I really don't know when or what happened, in this I am a real blond dummy.
Thanks anyway
Ciao
Simon