Either the breaker is overloaded, or, more likely, there is a loose connection at the Line or Load connections of the breaker.

A Main cannot be worked on safely without disconnecting power to the Line Side by removing the electric utility meter, which must be done by a qualified electrician or the power utility. Many utility company's do not allow electrician to remove the meter.

The heat created by a loose connection probably damaged the breaker, and a replacement will be needed.

Call in an electrician to troubleshoot and resolve the problem.

Don't be surprised if it is recommended that the whole panel be replaced. Usually, as TK stated, along with the breaker the buss is damaged by the heat and arcing if the problem is on the load side.