What House said.
Also it helps to think of the SIM card as a credit card. If someone stole your Visa and made a bunch of purchases before you called to inform them the card was missing, you would be responsible for those purchases.
The same rules are applicable to SIM cards. If someone else were to use it to call China for example, you would be responsible to pay for those calls simply because you didn't inform the phone company you lost the card.
Your best bet is to call them right now, and have it cancelled. Replacements are $10.