What’s wrong with my ATV?

I just bought a 07 Polaris Sportsman X2 800 about 3-4 weeks ago. Everything has worked fine.

One thing that happened today was I took my Uncle for a ride around our backyard, probably only rode it for about 5 minutes to show the property lines. We just went a normal cruising speed.

I then let him take it for a ride and he had it in low gear. He told me he came back and turn it off, but it was in neutral. I then went back to ride it again and when I put it in high gear and gave it some throttle, it didn’t really move as much. It felt like no matter how much throttle I gave it, it didn’t really move much. It probably only went to about 3k rpms. I tried L gear and back to H, turning it off and back on, but the throttle wasn’t doing much??? A couple of times, I let it idle, and it just died. A couple of times, I heard the exhaust pop.

What could be wrong?