Check that the control cable is still anchored by its clamp near the carburetor. It is not unheard of for the little clamp to loosen (or fall off completely) which renders the cable "useless" to pull the throttle back to idle speed.
If the cable is still secure near the carb, check the same thing on the upper end of cable near the control lever/knob on the dash.
If not either of those, you may have lost a throttle return spring from your control linkage.
Anything deeper than this stuff will require us knowing the exact brand engine and all its ID numbers.

Also check the carburetor for-
1. Throttle plate stuck wide open.
This can happen if an engine has sat for an extended time. The shaft can be gummed up, NOT allowing the governor to rotate the shaft and close the plate.
2. Throttle plate came off the shaft.