92 AstroEXT heating problems

I have a 92 Chevy AstroEXT and for a while my heat would not go on right away. If I left it on high or pounded on the dashboard it would eventually go on. But now it hasn't gone on at all. Any suggestions on what it may be and/or how much I'm looking at?

my friends have a 2000 and they had a problem with the air going cold after stopping the van and letting it idle - he checked and the coolent was low filled it and its been fine