First, in EVERY REQUEST FOR HELP potential responders NEED TO KNOW the brand name of the tractor (and model number), and the brand name and model number of the engine. For transmission troubles, we would also need to know the type (including manufacturer and model designation.) THIS IS INFORMATION THAT EVERYONE LOOKING FOR HELP MUST INCLUDE IN THEIR REQUEST.

Think about it - if you had a car problem, you wouldn't call a mechanic and say "I have a Chevrolet that won't start. What's wrong?" You KNOW that he would either simply tell you to have it towed in, or, if he was feeling generous, would ask a bunch of questions like: What KIND of Chevy? What engine? Automatic or standard? And so on.

In your case, and without complete information, I suggest that you remove and thoroughly clean and reattach BOTH ends of BOTH battery cables. Test and charge your battery. A great many lawn tractor problems are caused by corroded and/or loose electrical connections, and defective or weak batteries.

I hope I haven't sounded too harsh, but doggone it we've got to know whether we're dealing with an apple or an orange.

Would help a lot to know the brand, model, and age of the rider.

If you're getting a click when you turn the key then we can usually rule out a safety interlock problem.

As posted by 12ga, more than likely a bad battery or loose or corroded battery cables. Could even be loose engine mounting bolts.

How old is the battery?

Remove the battery and take it to an Auto Zone or similar where you can have it load tested. Check and clean both ends of both battery cables.

Make sure the starter bolts and wire connections are snug. BE CAREFUL, there's ample amps on some of those connections to hurt you and cause damage... even with a dead battery.

The mower is an MTD Model # 136C450F033. The engine itself is a Briggs & Stratton Model # 28D707 11.5 HP I/C I don't know how old it is. It came with my house. But the battery has been in good shape with no problems. The cables look fine too. As mentioned, it was running just fine until I shifted wrong and now it won't restart.

OP - as mentioned above, pull the battery. Take it to a qualified shop for a LOAD test. Meanwhile, clean the battery connections at both ends to bright metal with sandpaper or such and reassemble tightly. Reinstall known good battery and retest starting.

COULD BE a trans short with the battery. Make sure the 1-3 ground wires are tightly connected to the frame too. Silicon spray the shifter too just to be extra safe. Also wear rubber gloves and saftey glasses too.
The Hybrid Guru. John Sullender.

Make sure the PTO to mowing deck is turned off. I know that trying to start a tractor with the deck engaged will produce a noise (click, or clunk) when you try to start. no spark at the plug sort of cries out safety switch.

Just had the same issue with a 3yr. old Cub cadet. Barely hear the noise you're talking about. Don't have a clue. Too many electronics and switches

Depressed brake, applied voltage straight to the starter, engine cranked and did not start. Just about has to be the reverse/mower switch. Where is it on a Cub cadet lt42"? 3yr old. And how do I get to it?

Lights won't turn off, the faint noise lasts about 5seconds and comes from between the gas tank and dash