As none of us can see the inside of your PC, `big fan` and `little fan` don't mean much. Possibilities include PSU failure, CPU/CPU fan failure, motherboard problem. A fan about 3 inches across mounted directly on to the motherboard will be your CPU fan, if that's not moving then your CPU will most likely be damaged beyond repair. If you have internal fans (60-80mm) placed at air-vents in your case, they're not critical and can be easily replaced. If your PSU fan is not working that will spell trouble too.