boost gauge not working

my grandfather took his car to get the valve cover gaskets replaced they were leaking,well since he got it back the boost gauge will not work,i know its making boost you can hear it but it wont show any,i checked all the wires and theyre all hooked up what else do i need to check to fix this problem its a 98 grand prix gtp thx.

If it working before then it has to be related to the work done. I'd check all vacuum lines on the supercharger. Paying close attention to the one for the MAP sensor. Also, check the operation of the boost control solenoid. Check the electrical connections for the MAP sensor and boost control solenoid. Hard to tell you are making the proper boost and power by sound alone.