Most that I have seen do not have an internal breaker. What brand and model do you have? If he used to small of a cord (which is common) then it may have shorted out the armature. I see this happen with power tools quite often.
If the circuit breaker trips there are three possible reasons 1. the motor is being overloaded - clean underneath ( as recommended above); or if the grass is too wet or too long or both you may need to handcut; or the motor bearings need lubricating (same problem as me-how?)2. There is an electrical fault - check the plug and cord (as above) or get an electrician to check the motor and connections; or the carbon brushes to the motor need replacing3. You have a faulty circuit breaker! - borrow and test with a replacement