Overloaded circuit
Too many high-power devices on one circuit, like a space heater and hair dryer, is the most common cause. Move one to another circuit.
Short circuit or ground fault
If it trips the moment you reset it, or you see scorch marks, leave it off and call a licensed electrician.
Never force it
Don't tape a breaker on or swap in a bigger one. Breakers protect your wiring from overheating.
General information only, not legal or professional advice.