You could have the venturi blocked with something, or freeze damage; fairly likely. There could be something blocking the screen. You could have freeze damage below the venturi and it is sucking air; but you don't mention air so probably not. You could have less water in the well and the pump can't lift it very well. The impeller could have freeze damage. You could have a stuck closed foot valve. In other words, it's anyone's guess. You'll have to check some of these things out but remember that 15 years without problems has nothing to do with problems today except that 15 years says it's now time for repair.
Quality Water Associates