What happens if a mutual fund account goes down to $0? Does it then go in the negative?

Or do you have to put more money in at that point?

I was referring to MY account within the mutual fund; not the mutual fund per se.

You can not lose more then your original investment when buying stocks or mutual funds unless of course you're buying on margin which you shouldn't do IMO.

I have never heard of a mutual fund hit 0

A mutual fund usually has a diversity of investments in it's makeup that cannot hit zero, all of them at the same time.

But if it does, you do not have to put more money in

If a mutual fund's Net Asset Value goes to 0 it could either mean the fund company went broke, or they transferred the tranferred the fund to a different type. Such as TIAA-CREF did with some of theirs when they switched from retail accounts to to institutional retail. But since the NAV is the book value of the company, its securities and assets minus its liabilities, it could go to 0 or less. You would only lose what you have invested though.

Actually, the Ameritor Fund's NAV got down to less than 1 cent per share earlier this year. (How they managed to do that is a story in itself). That's as close to a zero balance as I've ever seen. I'm not sure that the fund still exists. I couldn't find a price quote for it when I just looked.

The value of the fund's securities cannot become negative because there is no such thing as a negative valuation for a security. Zero is as low as it gets.

No, you would not have to put more money in. In fact, you'd be crazy if you did. The fund would be basically bankrupt (even if not legally bankrupt) at that point.