Tech Drive
The Techdrive could be a number of things--a broken belt (rare), a mis-adjusted handle mount (where the handle connects to the machine) (most likely cause) or a bad transmission. Worst case, you can get a new tech drive for around 100 bucks, installation isn't too bad.
If the handle is the issue, that can be adjusted without too much difficulty. The techdrive is reliable, but as it ages it loses some force and has to be re-adjusted to have the same performace. This is done by loosening the handle mount and re-tightening it, noting at what point the clutch engages in forward and reverse.
When you put the drive pedal in "D", do you hear any sort of a whiny noise or a noise different than when the pedal is in "N"? If you hear nothing, it could mean the belt has either slipped off or broken (again, rare, but I have seen it) and a 2.00 part will fix it. I would start here, and then adjust the handle. If neither of those fix the situation, you have a bad transmission.