error establishing a database connection

I am using the coupon theme and from time to time I get this error message: error establishing a database connection. This is an error message I have always had when using this theme. I had been using the theme for a couple of years but then took the site down for a while. I have now started to use the theme again and I am now getting this message again. Does anyone know what I need to fix it? As I said it doesn’t do it all the time!