How to check the DB connections

Hi everyone, I am new to IT side. Joined as AWS support.

Suddenly The connections have increased to max level for the DB. All the sites and portals went down. We are still unable to fine the root cause for that. Normal connections to the DB is around 70 to 80. But it went to max level as 640+.
So how can I check who is using and what is using this connections.