Hi Robert,
If you are not getting a reading from either one of the senders, It could be the gauge that is bad. If it is just one sender that is not reading, you might try unscrewing it from the cylinder head, cleaning the threads and reinstalling it. If it doesn't have a good ground between the sender threads and cylinder head, it won't work. Don't worry about losing coolant. The sender doesn't actually contact the coolant, but screws into a cavity in the cylinder head where no coolant is present. You might check the cap on the sender that the spade connector snaps onto for corrosion. Anywhere else you have connections in the wiring could also be a problem. The signal transmitted through this wiring is so small that even the slightest resistance anywhere in the circuit can cause a malfunction. If it is just one side that isn't working, switch the positions of the two senders. If a sender is bad, the fault will switch to the other gauge. If the fault does not go to the other gauge, It's in the wiring or the gauge.
If it does prove to be a wiring problem, a little dielectric grease on all the connection points after they are cleaned will pretty much stop it from happening again.
Good luck,
Bill
