How can I fix my office chair?

Looks like the valve in the pneumatic cylinder is deffective, or the cylinder lost it's presurisation.

Repairing those require some heavy tools that you probably don't have.

You can take it apart and replace the cylinder. Those are prety standard.
Under the base, there is a simple metallic clip. Removing it can be done by hand and will detach the wheels assembly.
In some cases, it can be a bolt. Use a combination wrench to remove it.

The seat is secured in place with another bold. You must take off the seat, usualy by removing 4 screws to expose the bolt.
Use a wrench to remobe the bolt.

Get a new cylinder and reassemble.

Kuälinar
February 2012