I take it that you mean the fuel and temperature gauges...
There's a million and one posts on this- you need to go to the particular technical forum that pertains to your problem (in this case, Electrical and Ignition), click on "Search" in the upper right hand corner of the page, type in your search parameters, select "This Forum", and enter.
Sample results: ELEC--temp and fuel gauge out!! Help 1968 barracuda temp gauge and fuel level inop how to bench test a 70 Dart instrument cluster? Need help with fuel gauge not working?
It's most likely that your IVR (Instrument Voltage Regulator) has gone bad, but there can be other issues that pop up. Too much that has already been said that I don't have the time to reiterate here. Read up and pop back in with any questions that you may come up with!
