Do you mean it turns over but will not catch?
If the engine is turning over but won't start it is most likely a fuel problem. Could be a clogged filter, a bad fuel pump, air in the line, a broken off intake line in the tank, a cracked intake line, a bad fuel shutoff, water in the fuel, etc.
First thing I would do is check the fuel filter. See if there is air in it first. This would indicate the intake line is open somewhere. If there is no air in the filter then drain it into a clear jar and let it settle to see if there is water in it.