DB crashed

I have been using postgres since a long time. Recently number of users in my portal has been increased drastically and because of that load on Database server has been increased. Suddenly postgres Database crashed and I need to restart the DB.  I am not able to find out root cause of this problem. I need help to get any clue regarding what query caused DB to crash. Any utility/program that can help me to debug the DB will be well appreciated.

- R