Error establishing a database connection - Wordpress EEv4

All my websites are in the same server, and every 14 - 20 hours apears this error:

" Error establishing a database connection"

can anyone help me please?

Is the mysqld service running when you receive the error? Sounds like your server is running out of RAM after a period of time - the mysqld service is usually one of the biggest users of RAM and the first to crash when available RAM is low.