What could happen if I used 4 cycle fuel in a 2 cycle engine?
There is quite the extreme possibility there has been permanent damage done. 4 cycle engines do not need oil mixed with the gas to provide lubrication, since it has oil in the crankcase.
Two cycle engines have to have oil mixed in the gas to provide the lubrication needed, since there is no oil in the sump. When you used the 4 cycle gas, there was no much needed oil mixed with the gas. There may be a small chance the engine has not seized, it may start again, but the life of the engine has been greatly reduced.