First, make sure it's a legitimate bill. Many vendors send communications that look like official bills or vendor invoices and are nothing of the sort.
If it's a legitimate bill, challenge it with your credit card company. Make your credit card company show you proof your card was used for the transaction. You're protected from theft of, or fraud involving your credit card as long as you can prove it was stolen. The proof generally acceptable includes:
1) patterns that do not match previous card usage (geographic location, vendors, size of purchases, times, dates, and methods of purchase)
2) a police report
3) an insurance report
4) a record of previous communications with your credit card company reporting your card as stolen
Bank associations generally provide indemnity above $50, so if this is the only unauthorized use of your card, and you cannot prove your card was stolen, you may get to stuck for the bill. However, your first step is to challenge the charge with your credit card company. Do this immediately to preserve your rights.