Why doesn't my suspension fork rebound after being compressed?
Sounds like you have debries blocking the valve at the rebound chamber. Even if you have no air pressure in the chamber(that means 1 atm) it should still have enough force rise back again after being compressed.

I for once bought a "White Brothers DH-2" which was mainteined recently, or that's what they told me. One day it got compressed all the way(15cm+3cm kit) and got stuck. I opened the tubes very carefully, you know because springs. The "mechanich" sleeved the springs in a piece of rubber(in fact was a cuted out wheel tube) and with friction pieces started to come apart and made it's way to the rebound chamber, forming a plug.

Cleaned it up good, lubed up the seals and changed the oil. Stuff felt like new afterwards.