If you have a fork that uses air to provide the spring, losing pressure means the fork will get shorter and stop being responsive over bumps.
Most forks use a Schrader valve on the air chamber (the same as car tires). For low pressure, high volume forks (where you normally run below 100 PSI pressure), a tire pump can usually be used to at least partially inflate the fork. This won’t help if a seal is broken inside the fork, or on high pressure forks. It also relies upon the pump having clearance around the shock inflation valve.
You will find the inflation valve under one of the caps on the fork crown, underneath the fork legs, or for some Cannondale forks, either on the underside of the steerer tube or under a cap on the top of the steerer tube.
If re-inflation isn’t an option, or doesn’t work, use the fork’s lock-out (if it has one). You will be in for a rigid ride, but this is unlikely to be (any more) damaging to the bike.
Bikes with two air chambers (positive and negative) may benefit from a reduction in the negative air pressure if the positive air chamber is too low. The negative air chamber provides the downwards force to stop the fork from pushing up too fast, and an imbalance will cause the fork to be pushed into a compressed position. If you do reduce the negative air pressure to compensate for the low positive air chamber, the fork will feel very “soft.”
If the fork is leaking air slowly, try turning it upside down for a while. This can get the oil inside the fork to pour back around the seals. Strangely, storing your bike upside down longer term can have the opposite effect of making the air leak slowly.
Taking the fork apart on the trail is unlikely to lead to a fix – air chamber forks tend to be relatively unserviceable without specialist tools, and they can “get naked” fast – meaning, lots of little parts can end up everywhere with no hope of getting them all back in again. If you must take it apart, depressurize all the air chambers first.
Tip: Forks normally have a “bottoming out” damper – basically a bit of rubber to stop them from compressing too much. So even if the fork is completely compressed you probably won’t damage it any further.
Tip: With any depressurized fork, check front tire clearance with the fork crown before you ride. Having the tire catch on the crown when you are moving can bring the bike to a stop pretty fast.
Tip: Suspension components are one of the most non-standard areas of bikes these days. It really pays to carry the pump and special adapter fittings you need for your fork with you.