I have a 2006 Cub Cadet Tank with only around 350 hours on it. I bought it used from a guy about a month ago. He had a guy do some regular maintenance on it, oil, spindles, filters, etc. He never used it commercially. Today when I mowed though, I was about halfway into my 3 acre flat field, when the power started fluctuating, so I turned it off in a panic. When I went back to start it, It would turn over, but not start. I tried starter fluid, but it still did not start, though it sounded like it wanted to. Got a little white smoke, but that is relatively common for a hot machine. About 2 days before the problem, we had a small oil leak from the filter, so we topped it off with some 30 weight Non-Detergent oil. This was the first time it was started with some different oil in it, but we only put a small amount in it, so it shouldn't be a factor in why it will not start
May have hit an obstacle and sheared the timing pin (jumped time). Have a look at the top of the flywheel and see if the shear pin is lined up with the slot.
