The heater does absolutely nothing when you turn it on. Hit the switch and nothing happens. No noise from the pilot light, no flame, nothing. Its a Raypak Versa that's approximately 23 years old. Is it repairable or do I trash it.?

Unless there is water running out of the inside it is always repairable. You can start by pulling the burner tray and cleaning the unit and the burners. Also pull the wires and clean the leads and contacts. At 23 the heater is probally a milivolt run heater so a small drop in power can stop the heater from working. Often the problem will be the pilot generator or the gas valve. The generator should delevop 1/2 a volt to run the heater. This can be measured with a milivolt meter. You should get 500-700mv from the generator. If you get that and have no pilot you may need a gas valve and if not you probally need a new pilot generator and it would be best to replace the whole pilot assembly. If you live in Calif. or someplace that makes standing pilot obsolet you can probally find these parts on line through Ray-Pak.
But before starting check the filter to be sure it's clean. If you have had the pool for 23 years that goes without saying but I have to say it anyway.
After that you can use a jump wire and isolate each itim in the saftey circut (thermostat, high limits, presure switch...) just follow the wire when it leaves the gas valve and follow it to each item and jump across. If you find one of these itims to be the culpret it is just a simptom and their is probally another problem and the part just keeps the heater from working so as to keep it safe from that problem. Good luck and let me know what you find and we can go further if need be.