Make sure the engine works fine... air filter, spark plugs, oil, GOOD GASOLINE, even the safety switches. I have had this happen before and I just checked the starter with a multi meter while the ignition is ON, then worked up the chain, solenoid and so on. I have even had the ignition switches go bad. Then I just got a new starter and "hot wired" the mower to life. If there is NO click when you turn the ignition, test the solenoid, then work all the electronics, seat, brake and PTO sensors first, then the rest. I am crazy, but not dumb....I feel it is an electrical problem...what I do not know, but I am not a multi meter. GOOD LUCK!!!! Make sure you reference the both the troy-bilt and the B&S manuals. the troy-bilt manual will most likely have an electrical diagram from the entire chassis to the starter of the B&S engine. Good luck and a multi-meter is your friend!