Lukather.....Have had this with a couple of GTVs; Take the bottom of the cowling off and find the block connector. (if you take off the panel with the fuse cover you might see it without taking the cowling off)
Make sure the battery is disconnected first!
Take the block connector apart and spray liberally with contact/electrical cleaner.
Leave it for a few minutes to dry out, put it back together, reconnect the battery, do the old MAR re-set on the ignition (make sure EVERYTHING is off including the radio and internal/courtesy lights - so lean in through the window)
If this doesn't work then the ignition switch has probably slipped a spring.....you can take them apart and repair them but;A....it won't last long and B, you need to find an 8 year old, Asian Gynaecologist with tiny hands and 6/6 vision to put the springs back!