From what you've written the most likely problem seems to be something wrong with the input to the Peavey head. If your spare amp has a preamp out and the Peavey has a power amp in (or effects loop send and return, respectively), then a good test would be to plug the guitar into the input of the spare amp, then connect the pre out/effect send from the spare amp to the power amp in/effects return on the Peavey and see if you get noise from the Peavey through the Orange cab.
If you do get sound that way, then most likely there's a problem with the Peavey preamp. If it's a tube amp, then you could try replacing the preamp tubes and see if that helps at all. If it doesn't help or it's not a tube amp, you might have to take it to a tech.
If you don't get sound with the above test, then you could reverse the test and see what happens. Run the guitar into the input on the Peavey, come out of the preamp out from the Peavey into the power amp in on the spare amp, and see if you get sound. If you do, then and the Peavey is a tube amp, you could replace the power tubes and the phase inverter tube (which will look like one of the preamp tubes - the manual should specify which tube it is). You also should check that you are using a speaker cable, not an instrument cable, between the amp head and the speaker cabinet. And you should check that the nominal impedance of the speaker cabinet (should be written on the back or available online - the "ohms") is equal to or greater than the rated output impedance of the Peavey (again, written near the speaker out or available online).
Amps can be really hard to DIY on, and usually dangerous, since very high voltages can be present. If none of the above works, you pretty much have to take it to a tech.