Your system is probably not able to get to the database, which may be on another PC or a server.
Windows often "falls asleep" with it's connections to other PC's, and you may need to "wake the connection up".
To do this open up "My Computer", look for the O: drive, and double click it. If you then see data folders and files, you have successfully "woken up" the connection.
If you get an error message, then the PC with your data may be unavailable - check the server or other PC to ensure it is switched on.
You may also need to check the cabling to ensure the PC is physically connected to your PC.
Once you have a good connection to your O: drive, try the Listings system again.
If it still gives an error message, look for an icon on your desktop called "Repair Listings Database". Ensure all users are out of the system, then run this repair. If it succeeds try the listings system again. If not, call Realbase for further support.