Why is my 2000 Dodge Grand Caravan running hot?

[Momof3]

I have a 2000 Dodge Grand Caravan Sport SE V6 3.3L. The other day, I'd noticed it was running hot, so I got out and checked and noticed it had no water. I put water in it and it was fine. Then, about 4 days later, the same thing happened. I've also noticed that when it doesn't have water in it, the heater doesn't work. The oil looks fine and it looks like normal exhaust coming out of the tailpipe. Does anyone have any idea what this could be?