If the Renards aren't getting a signal, check to make sure that your transmitting XBee is getting power and that the receiving XBees are also getting power. (Use your DVM to measure across pins 1 and 10 on the Xbee -- it should be 3.3vdc or so.) I've never seen an XBee or a PIC to lose its firmware, so I doubt that's the problem. I have XBees that I flashed in 2009 that still work perfectly after being in a box on the shelf all that time. Remember that the XBee adapter (if it's the serial one that plugs into the SS board's serial input, it also needs to be connected to +5v on the SS board)
If you're using ESP modules instead of XBees, you of course need the show's WiFi access point running or the ESP's won't do anything. ESP's usually have a status LED that's visible when the module is powered on. (If the ESP can't find its network, it seems to take a trip to Neverneverland...)
If the SS board's 18.432mhz oscillator is dead, you'd need either a DIGWDF SST to check for the oscillator's pulse or an oscilloscope. I have seen a couple oscillators go bad over the past dozen years but it's pretty rare, too...