The Nav system controls all display on the bordmonitor, but has little to do with the gauge cluster.

It sounds like you are referring to your monitor up front, in which case, the most likely cause of no/bad display (in order of likelihood) are:
1. Weak battery
2. improper wiring of the nav unit
3. bad nav unit

If you had someone change out your battery / alternator there is a good chance that they didn't wait for the nav unit to power down before disconnecting the battery - this is a known cause of blown nav units.

d-