What should I do? no Forward no reverse

My 2006 TRX 400 FA Model 2778 gears do not work
Maintenance: Change oil, differential and gear levels are good and maintained. Use premium gas
1. Ride less than 25 hours a year.
2. 2015 gear would not go in reverse unless engine was warmed up and run in neutral for 10 minutes.
3. Check all fluids and and nothing was low.
4. Rode at an quad park outing this July for 6 hours everything worked. Except for when first started wouldn’t go in reverse until warmed up.
5. Twice the quad got buried in a water/mud ditch. Stalled out but pulled out and started up. Cleaned air filter at camp and ran great.
6. Rode in September for 20 minutes to keep battery charged. No problem.
7. Now it wont go in drive or reverse. Tried in manual shift and also in 4 wheel drive. Nothing.
8. I can do basic repairs but pulling gears and transmission way over my head.
9. It has 132 hours and 480 miles on it.
10. The oil light comes on to change for maintenance.
What should I do? If it cost me $500 or $1,000 to repair. I will have to turn around and sell it to pay off my debt.