That's a different cause/effect. Probably just the sender. Pull the signal wire off the sender, aligator clip it to ground, turn ignition switch on. If pointer races to the max hot position, turn the switch off and replace the sender.
Done.
If the pointer doesn't move, you can try to reach in from underneath and wiggle the gauge by its contact posts. If those nuts are a little loose the wiggle should cause contact a some pointer movement. Then you'll know that the gauge works at least. Then to tighten those 2 nuts would solve that one problem but... The fact that there was a problem in that one suggests potential for other problems that should be addressed.
I've found the best method to the madness is just pull the panel, service the entirity once and be done with it for a very long time.
