First of all, tell us about your setup in terms of plaform, hardware, setup, etc...
Have the databases been optimized on a regular basis?
What is the database type (innodb or others)
what database is it?
lemp/lamp or windows stack?
Have you attempted this before? If so when? where there any changes?
What is your OD/PHP version?
Got caching installed?
Separated app and db server?
Any runaway process?
Did you try and monitor the server with tools? Anything stands out?