website opening problem

sometimes my website get works and most of time not.
sometimes it show this msg.

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.Please contact the server administrator, [email protected] and inform them of the time the error occurred, and anything you might have done that may have caused the error.More information about this error may be available in the server error log.Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Sometime this

Error establishing a database connection
This either means that the username and password information in your wp-config.php file is incorrect or we can’t contact the database server at localhost. This could mean your host’s database server is down.
?Are you sure you have the correct username and password?
?Are you sure that you have typed the correct hostname?
?Are you sure that the database server is running?
If you’re unsure what these terms mean you should probably contact your host. If you still need help you can always visit the WordPress Support Forums.

dont wants happened to my website. i delete my previous version website and install wordpress. but still same issue. someone tell me may be it is incompatibility your wordpress PHP version and server PHP version.

Please help me
my boss fired me