If by "money" you mean the silver colored coins, that's plat and you have to buy it. If it is the gold colored coins, I don't know why it would go to zero except if you spent it. When you blow up chests you see a yellow number sometimes with a + sign, that means you just got gold.

Test it thus:

1. Write down the gold and platinum numbers.
2. Go to Forest Haven I and lock it.
3. Do the level and blow up all the chests along the way.
4. Look at your money.

There should still be zero in the platinum, but the gold should have increased. If not, come back here and tell us.