is it cold weather?is there anti freeze in it mate?
the reason i ask is the exact same happened to mein january in the wifes e46 touring, temp went right into red limped 3/4 miles to the nearest garage he had a tester to test the water temp! anyhow apparently the bottom half of the rad was a frozen block of ice!
put in 3 litres of pure anti freeze,and before anyone jumps the gun
i,m in sweden and when temps hit upto -38/40 we have special anti freeze anyhow left it for an hour and it ran,and still does,100%
hope this helps mate