We've had 2 instances with our 18 yr old well quiting. The first was the celenoid(sp?) in the pump house and my husband was able to replace on his own. It was leaking oil and that's how we knew it was the problem. Cost about $50 for the replacement.
The second time was more expensive and had to have a guy come out and replace the pump in the well (100' down). That cost about $1200!
Luckily, the well wasn't dry! Had a cousin over near Spokane, WA that had that problem a couple times. Needed to use a pump truck filled with water.
Good Luck!! Hope it's not an expensive fix.