We'd need more info on what type of motor it is, but in general these things are really simple. Usually in the recoil mechanism there is some sort of spring loaded sprag that catches a slot in the crankshaft or flywheel. This is broken or not catching on yours. You'll need to take off the recoil starter and probably the flywheel cover to see what is going on. It should be fairily obvious how the thing is supposed to work, unless parts are missing or disentigrated.