Error establishing a connection to the database

HI,
I’ve migrated my site from one to domain to another and since i’ve moved it i get the error message ‘error establishing a connection to the database’.
I’ve moved wordpress sites before but never encountered this problem.
It’s now on an apache server (not sure if that makes a difference?) and i haven’t changed anything in the wp-config file so theoretically it should still work – its just that all the links would still point to the old domain until i do a search and replace.
Has anyone encountered this problem before or know what may be causing it?
Any advice would be appreciated.