My L100 tractor was working fine but after I removed the deck to replace a spindle and blades I believed I had replaced everything back where it belonged but when I went to start it I got no response. There is no clicking or starter turning or anything.
I checked all the wires, the fuse and everything else I could see but nothing jumped out and told me the problem. My initial thought is the seat sensor but I have no idea how to check it. I can find it, unplug it and put it back but that's it.
Any help would be appreciated.

Check your lever that engages the pto /mower deck make sure its all the way off.

Sure it could be the seat sensor (safety switch) but it is far more likely to be something you didn't quite get right in reassembly. Start with DGs suggestion.