L2800 Won't start

Did you check the fuses? If the L2800 is like the L2900, the fuse panel is on the left side of the instrument panel just above the brake pedal. There is a "slow blow" fuse that protects everything - which may be blown. Check them all.
Good luck.

Have you cleaned the battery terminals? My 9000 did what you describe after being run all day cutting hay. Shut it off and it wouldn't crank back, or do anything for that matter. Cleaned the terminals and all was good.

When you're jumping the starter, you're bypassing EVERY starting system control....key switch, neutral switch, PTO switch, everything. When you're jumping the starter like that, all you're doing is testing the starter and possibly the battery. So you know those two are "ok", time to dig a little deeper. The starter is controlled by a little white wire. It's powered through all of the switches. On a gear drive tractor, the PTO switch, range gear shift switches are the only two switches that will keep it from starting. HST, you have a clutch safety switch and a HST neutral switch. You'll want to test each switch. A cordless test light makes it really easy. They have two wires per switch. In the "neutral" position, there should be power on both wires. Real simple system. IIRC, the gear drive tractors have TWO switches on the range gear shift lever and they both have to be pushed at the same time. If you look at the shifter you'll see them, and both have to be working. If one dies, it will not crank over because the tractor "thinks" that it's in gear. It also won't start if one of the switches-or wiring-is not correct, even by jumping the starter, because the fuel cut solenoid also works off of the switches. When you turn the key to "on" you should hear an audible "clack"-assuming the PTO switch is "neutral" and neutral switch and wiring are correct.

There is also a fuel stop solenoid fuse and I have seen the fuse die on a couple L3400's (same as the L2800). I was told by a reliable source that the fuses were junk. Didn't make any sense to me but on my boss's tractor (L3400HST), the fuel stop solenoid fuse failed, he replaced it with an auto parts store fuse, and it never once had another problem.