A few basic questions first:
Is is correct that the gauge worked before the batteries were replaced?
When you say no bars, I assume you have the digital gauge. That should go through self test when you power up. All segments with display. If not and it worked before, you probably have no power. If you have two guages, spedo and tack, both should go through power up self test.
If all these things worked before, you messed something up with the battery swap. One of the more common mistakes is leaving off the short black cable that ties the two batteries together. Check that there are no loose smaller size black cables laying in the bottom of the battery compartment. That will be the accessory master ground.
You may have also inadvertently popped the accessory breaker that is near the batteries and has a red reset button on it.
My bet is that you find a cable disconnected near the batteries.