how can i fix my sprinter 311cdi heating??

hi a couple of days ago i went to turn the heat up on my '03 plate swb sprinter van and the heater knob/dial nows turns all the way around instead of stopping at the top(hot)... all it does now is blows out cold air can anyone please help me it's freezin in the mornin, i am capable of doin the job myself i just need to know where to start and what to do.

Thanks in advance