Error in establishing database connection

We have hosted a website and it worked normally for last four days. Yesterday we faced a bandwidth error in terms of server and server side guys fixed this error. And the site back to normal stage.

Today it showing an error message ‘Error in establishing database connection’, hence, if we refresh the page the site appear normal, after refreshing the page again it’ll show same error again. This error is appears and disappears in the wp-admin area as well.

If anyone could make us a solution for this to fix this error. Please help on this issue. What may this problem.