No heat. The AC works fine; I checked the breaker box. The heat came on a couple of times

Question

The AC works fine; I checked the breaker box.  The heat came on a couple of times for 10 min or so in the last week but now it won't come on at all.  The fan works in the ON position but no heat.  Can you give me some idea of what the problem might be and cost to repair before I have to call someone to look at it.  It was manufactured 4/2003.  Thank you.