Error establishing a database connection

Since I updated to the latest version I’m having problems. First brought to my attention Tuesday evening by a client. When they tried to access my blog they got the error message in the topic title. I was amazed as just minutes before had logged into wp-admin to add a new post. About 10 mins later it was working again and I finished my latest post. Job done. Until a supplier mentioned it happening yesterday (but I didn’t have any problem bringing the page up just 2 minutes later). And now it’s down again…and I can’t access the control panel. When I go to wp-admin I get this message:

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.

I haven’t made any alterations and can’t see any problems – my main site (non wordpress and hosted on the same domain) is working fine so it’s not a hosting problem.

Help!