I assume you are getting this error message.
This message appears if someone else with FileMaker Pro is using the same license key as the copy you are trying to launch. Pull down the FileMaker Pro menu and select "About FileMaker Pro" and click the Info button. This will show your license key.
Next, go to another machine that is running FileMaker Pro, and check the license key of that user. Keep doing this until you find the other user.