00' XR400 No Compression

My first guess is you have a valve stuck open. That should be pretty easy to check.

2nd. guess (which just happened to my bike) is your top end is worn out. On mine a ring acutually broke and I had very, very little compression. Unless you've ridden a ton, I doubt this is the case on your 2000.

on the dooooh list, make sure your spark plug didn't work its way loose. good luck.

Did you just remove the external cable to the decompressor, or is the decompressor removed from the camshaft?

If the decomp is still on the cam, then it's possible there's a bit of crud in the decomp on the cam that's holding the lobe out and keeping the exhaust valve open.

Do you still have the manual lever attached? If so, try actuating that a few times and kicking through to see if you can get things 'unstuck'.

If this doesn't do the trick, pull off the valve caps and take a look. Kick through a few times and see if one of the valves is not completely closing. This will be just a tiny difference, so rotate through slowly and look carefully.

If you can't find evidence of a stuck valve, it could be stuck rings.

If you rode the bike, then washed it while it was still warm, then put it away without re-starting, it can cause internal condensation and the rings could be rusted.

Basically, it's either rings or valves.