I changed my thermostat to a digital RTH110B.At first the compressor would work but the fan wouldn't.This morning the fan came on but the compressor or heat would not work.My central air and heat unit is a Ruud.It has a honeywell control on it.It has a gas heater.The thermostat is wired to my(4) wires by color to letter.HELP!

It might be time to call in a service person, without being able to put a volt meter on the system and trace out the problem it is just quess work at this time, if you have checked all your control wires for being tight and the thermostat is wired correctly then its time you call in someone, good luck.

You have a low voltage terminal strip somewhere on your furnace. You have to match the color of the wires to the terminals on this strip to the wires and the terminals in the thermostat.
The wire on the R terminal at the furnace will go on the Rh at the thermostat
Then the same for the W, G, and Y.
Then you will have two wires coming from the condensing unit to the furnace terminal. One should connect to the same terminal as the Y from the thermostat, the other wire will connect to the C, or Com Terminal at the furnace. It is the common from the transformer.

Basically it is
Red to Rh
Wite to W
Green to G
Yellow to Y

You may not have wires that match these colors, that is why you have to match them to the wires on the furnace.