temp gauge

ok this is what happen .I drove from home to work like every day when i got to work , i saw i bit of radiator water coming from my x5, not much just a small spot.i did'nt do anything. the next day when going to work about 20min into driving the x5 strated to smoke from the front and my temp gauge went from half way to red zone. now i stoped and called a toll truck and took it to my job ,ware i have a teck check to see what happen to onver heat,when it was checked by the teck he found that the top radiater hose was broken so the radiator blue water came out, that way it onver heated. now the problem that i still need help with is that my temp gauge only comes down to cold is when i trune off the x5,when i put in the key and trune it ,the temp gauge goes back up to red even if the x5 is cold or just stared,next to that every thing else is ok put iam not useing it untill someone tells me ,if is the gauge i have to fix or the temp sensor. please respon to me . thank you. x5