Question Heater stopped blowing hot after alternator belt change

Hi all and apologies if this has been asked before. I did read through some of the heater problems but thought mine was a little different so any help would be greatly appreciated.
I have a P reg diesel auto with a digital heater display. My alternator belt recently snapped so I replaced it myself which didn't seem like a problem until I tried the heater yesterday. Before the belt snapped the heater worked but only on 32 degrees which got very hot and if I turned it down to even 31 it would just blow cold. Now It doesn't blow hot at all. I am no mechanic but I will try and answer any things if you need to ask anything.
Thanks for any help in advance

It's seems like a possible airlock in the system if it's not a fuse which would be the first thing you'd have checked already. If your temp gauge shoots up then it's definite air lock. The easiest way to bleed the system is suck the air out of the long hose clipped under the passenger seat, with a hand syphon pump and return the water straight into the coolant tank. best of luck. kev

Hi
Just an update...I turned the engine on and topped the radiator and coolant tanks up. I then drove to Halfords to buy a hand syphon pump and antifreeze. On the way back home I heard a noise come from the alternator area and thought the belt had snapped again but instead the heating come back on again. Strange but I won't argue.
Thank you