I want to cancel the reservation on my hotel. And they say that they'll charge me with €900, but I don't have any money on my card. Can they do it?
The consequences (for you) depend what kind of card it is (is it a pre-paid card, a debit/check card or a credit card); however as a simple matter if the card has no balance - and no "overcharge" feature, the request will be rejected with a notice stating the same.
Keep in mind that they may have pre-authorized the charge (also called a "hold") if so, the money is effectively blocked for them and in this case they can still receive the funds.
If you are in doubt, you can check with your bank or card issuer.