If your credit report has been stolen from your mail, then that puts you at a risk of identity theft. You can put a fraud alert on your report at no extra cost by contacting all the three credit bureaus, Experian, TransUnion and Equifax.
A fraud alert will notify the lender that the consumer is at the risk of credit fraud and will take extra measures to prevent his from happening such as contacting you personally or asking that additional documents to prove the identity be provided.
It also matter whether the credit report that you are referring to was sent to you by the credit bureau or by one of your lenders. In some cases, such as mortgage lending, the creditor is required to share the credit report with the borrower. It is normally advised that a consumer gets his credit report from the credit bureau. This is because a credit bureau personal report camouflages certain information in your credit report. Your full social security numbers as well as your account numbers are truncated. This is because it is presumed that you will be able to identify your own accounts using the few digits that are mentioned there. So a personal copy of your credit report from the credit bureau is more secure as it is useless to an identity thief save for providing your name and address which is mostly common information that can be located in a phonebook anyway.
If your lender dispatched the copy to you then you will have to clarify with him as to how much information was revealed in the credit report. The copies that a vendor gets from the credit bureau and other specialized services is different than the ones that you usually get personally.