For the last couple of days at various times when checking my site this message keeps coming up

Sorry!
There seems to be a problem connecting to our database. Please give us a few minutes to remedy the problem. Thank you.

To verify the site status ... Click here

Why is this, as it seems to be appearing more and more regularly and its not just the admin site, no one can view the site, nor can I recieve emails etc....

What are the problems and why please ?????