This is normally either a broken mechanism or dislodged mechanism.
Isolate the power to the appliance.
(If it's integrated, you'll need to take it out of it's position).
Take the cover that goes up the sides and over the top off.
Be careful, microwave ovens can store a charge of over 4000 volts, even when unplugged.
Touch nothing electrical inside.
You'll see the push button, when pushed, operates the levers that opens the door.
If broken, you'll need new parts.
If dislodged, refit the parts correctly and it should open again.
Hope this helps