Refer to figure.
The altimeter is an instrument that is designed to measure static pressure and, using the conditions of the standard atmosphere, convert that pressure into a value of altitude.

A simple or non-sensitive altimeter comprises of a partially evacuated barometric (aneroid) capsule, a leaf spring, a mechanical linkage and a pointer

These components are all housed in a container, which is supplied with static pressure from the static vent system.

If the static tube or vents become blocked, the pressure within the instrument case remains constant and the altimeter continues to indicate the altitude of the aeroplane when the blockage occurred.