If the post was functioning properly. Your protective boot may have been dislodged from the upper ridge (item #58). Push down hard on the saddle while pushing the switch and see if you can get it to disengage. If you can get it to disengage, go to step 2. If you cannot, got to step 3.
It is also possible that your Top Cap (item #52) has become too loose. You can check this by locking the post in the up position and checking for up and down play. Push down on the seat and pull up. If you have a fair amount of play up and down, then tighten down the cap just until there is a slight amount of play. See “Proper positioning of Top Cap #52” in the Maintenance section.
Check to make sure the switch is working properly. Some cable stretch might have developed not allowing the pin to disengage. This has happened if you have cable slack located in the switch body. Loosen the set screws (item #29) with a .050 american allen wrench and tighten the cable. Be careful not to pull too much. This will cause the post not to lock in any position.