Fan on Radiator is not coming on anymore

bad fues, bad relay(you did use one didnt you?), bad sensor, or bad wiring, usually a bad ground, can all cause the problem you are having, along with a bad fan motor.

one little interesting thing from my recent e-fan conversion....make sure your ground is PERFECT. I thought grounding my fans to the frame would be sufficient, but whenever they would kick in, i would see backfeeding through my intake via a 20* jump in temp on the gauge. Ran my grounds to the batt and it completely solved the issue.

if they don't come on at ALL I'm assuming its a bad relay or fuse. if those are okay then it could be the thermristor. Just try and troubleshooot some and you'll find it pretty quick.