Need some help please guys - My 66 isn't starting...

[Neatbeard]

Please bear with me as i'm a newbie to C10's and american motors in general (but not classic cars a whole previously being a VW man).

I haven't had my truck long and the previous owner never had any problems, and the last time I started it up I didn't either.... all ive done is spray some WD40 around over the battery terminals and wire connections in the engine bay to help in our damp climate here in the UK. But for some reason the truck won't start now??

I don't get a 'click' from the starter motor, nothing...?

The dash lights all come on as they should but when you turn the ignition key to the next 'stop' where it should fire up nothing...?

The compressor for my air bags doesn't fire into life either (its on the ignition circuit)...?

I've put my jumper/starter/portable power pack on thinking it might be the battery is discharged but no....still nothing??

I don't get it all i've done is spray the WD40 which i've done on countless cars i've owned before and i've NEVER had this happen before?

Any ideas guys am i missing something obvious? This is doing my head in as i've not had the truck very long and I haven't had a chance to cruise in it properly yet.