Many office chairs are fully adjustable, allowing users to change the height of the chair. This is accomplished using a gas spring. If that spring breaks, the chair may sink to its lowest level or become frozen in place.
A gas spring is a sealed cylinder with a piston attached to a rod inside. When pressure is put on the rod, such as when somebody sits in the chair, the piston compresses the gas in the cylinder. This allows the chair to sink or rise depending on the amount of pressure on the chair. The piston can then be locked into place at the desired height.
If an office chair is frozen into place, will not rise on its own or sinks to its lowest level, the gas spring is defective or damaged. There could be a leak in the cylinder allowing the gas inside to escape or the piston, or piston rod, may be damaged, locking the gas spring.
For many office chairs, the gas spring can be replaced when it has become damaged. Contact the manufacturer of your office chair to inquiry about a replacement spring.