Lawn mower "bang", surge
If you were riding it when it started surging and the throttle is still work, I would guess that the choke cable either broke or the choke return spring fell off.  The choke plate will be on the air-filter side of the throttle, so look and see if it is flapping around.  If it is, then check the choke cable and return spring.
What is the make and model of the engine?  Do you have any photos?