Problem is, could be any number of things. A competent mechanic shouldn't have too much trouble sorting it though. The basic tests are to check for a spark at the plugs, and to check fuel is getting to the carb(s). If both are happening, then it will run... If it's switched on and getting power (i.e. lights work) but makes a clicking sound when turned over, suspect the battery/charging system. No lights means no power, so the battery and/or electrical connections must be faulty... If it's switched on and getting power but totally dead on the starter button, then it's probably a fault somewhere in the starter motor or solenoid. It may be blowing fuses if it runs, then dies again suddenly. I think that's a faulty solenoid, but I'm not much of a mechanic so someone may know more than me there...