hmm, maybe the low speed setting on fan or switch has packed in?
if you disconnect the fan switch if it has 3 pins its a dual speed setup. bridge the red wire to 1 pin, then the other and it should come on slow & fast. if so the switch may be at fault, or you have poor coolant circulation thru the rad. if it doesnt come on slow or fast, jump the fan direct from the battery. if it still doesnt come on slow or fast then the fan itself is buggered.