The default location for storing the virtual machines is ~/Documents/VirtualMachines.
You must have moved Windows 7.vmwarevm file to a different location.
If you remember moving this file to another location, you must manually navigate to the new location and open that file ( Windows 7.vmwarevm ) . Fusion will create a new entry in library and you will be able to access Windows 7.
If you do not remember the location, try searching for Windows 7 or Windows 7.vmwarevm in spotlight.