I have a 2000 Ford Expedition and it won't start. The red light (that looks like an oil lamp) went on.?

So the red light always comes on and my "quick fix" to this was to add water to the white plastic container bec. i have a leak i have to do this every 2 days. one day the light came on again and i couldn't pull over to a gas station to put water. i was about to exit the freeway and i turned off my car (bec. at this point my car was driving slow already). when i tried to turn my car back on when the light turned green, my car wouldn't start. my husband bought water and poured it in the radiator container (or some container on the right side of the engine) but that didn't start the car. my cousin said it could be the battery, but my husband just had it recharged at autozone and my car still wouldn't start. he suggested that it could be the alternator or radiator. before i spend money for towing it to a car repair shop, can someone else tell us for us sure what it could be? i guess i just want to know what i'll be dealing with so i'm prepared for the budget. Any info is appreciated