I've been there too
1. Keep Calm.
2. Carry on - and get yer multimeter (or sidelight bulb and holder on longish wires) out and verify that it reads / indicates when you put it across a known good battery.
3. Start at the battery and test accross it. (My guess is you'll have juice here).
4. Test the end of the big red wire at the starter solenoid, with the other test lead on a clean bit of engine. You probably have no juice. If this is true, you've possibly burnt the transmission / engine to chassis earth strap or the battery lead out.
5. The battery negative should still be connected to the gearbox (accessible / visible from underneath) You can test this item with a resistance setting, from battery negative to the top of the transfer box, under the middle seat.
6. IMHO, bodgers rarely add safety features (like pricey 300A fuses) to wiring, but if you trace the big red wire betwixt battery and solenoid (it normally runs in a bit of ribbed plastic ducting) you might prove me wrong...
7. You can get to both ends of every wire, so go round and tick of every connection in the starter circuit as you test them with the meter. Calm and methodical, and consider irate violence only when all else fails
Good luck!