A lost checkbook can be a serious matter, because there's always the chance that you may be held liable for any checks passed in your name. Contact your bank as soon as you know that you've lost your checkbook, and follow the financial institution's rules for dealing with the matter to the letter.
Place a Stop
You can have your bank place a stop payment order a certain group of your checks. To do this, you need to know the numbers on the checks that were stolen. Your bank may charge you a fee for the stop payment order.
Close Your Account
The other basic option is to close your account and open a new one. While this process can be somewhat time consuming -- largely because you'll need to transfer all of your auto-debits to the new account -- it ensures that no money will be stolen out of your account due to the checkbook loss.
Report the Loss
It's also a good idea to call the police and report the checks as stolen. This reduces your risk of being held financially responsible for fraudulent checks written on your account, which is considered a criminal offense.