Hi, not much to go on here and that includes whether it's a 2 or 4 stroke engine you've got, but first you said fuel which i could take as meaning diesel,in which case i would drain & flush the fuel tank & tap, strip the carb & clean, take head & barrel off and check/replace rings, as it hasn't run you may be lucky to get away with a clean.But if you mean "wrong petrol"? as there is only unleaded in either 95 or,i think, 97 octane , you cannot go wrong whichever one you use. i can only think that you have a 2 stroke fuel/oil mix in the tank type, but failed to add 2T oil & the engine has seized, but you say it wont start so it cannot really seize up by trying to start it!? check the spark plug anyway for fouling.
Chris P.Duck
December 2010