1998 a6 overheating, help!

ohey guys, i have a 1998 v6 2,8 a6 quattro, and i was comin back on the motorway to burton (yes where al the beer is made, go on chek yer beer cans!) and all of a sudden i get a warnin sayin the coolant temp is too high and the engine is overheating, so i pull over and wait for the car to cool down. pop the bonnet and the coolant jus disapears, so i put water in and carry on wen the car is cool. mile later same issue. end up havin to get a flatbed to take me home after waitin two and a half hours in the cold! now the car sill over heats even tho oil and coolant levels are fine, and the coolant doesnt disapear. even overheats wen i turn the engine on and leave it stationary. anys ideas on whats wrong with my baby?