Ok, im gonna do some guessing work now...
First of all, make sure your bottle has some gas in, try getting it filled.
Now the guessing starts...If you have a tippmann 98, clean off all the oil, check for anything thats not supposed to be there, then re apply a thin layer of oil to it. If that doesn't help, bring the back cap out to the second line and test it, if its that have you have changed the main spring? If that works take out the spring and cut off 1-2 coils, then try again. Repeat until it works. You may also want to check the O rings throughout the gun and replace if necessary, this will be another major cause.
Now, if you have an inferno, try taking out the bolt, cleaning it and the breech ect and check the o-rings and re apply the oil, this goes for spyders too.
-Philip
(Wannabe Tech)