This means you've either lost your mapping to your network drive letter, or that connection isn't established yet (you may have logged in before the "server" computer was turned on).
From your Windows Desktop, right click your CMS for Windows icon and go to the Shortcut Tab.
Look at the Target for the Working Directory setting of -W. What setting directly follows the -W, i.e. -W Z:\CMS4WIN\ Then you'll know your network drive letter is the Z: drive. Click OK.
From your Windows Desktop, double click My Computer and look for your F: drive.
If it's there with an X through it, double click the F: drive (or whatever drive is your network drive letter) to attempt to reconnect to this network drive letter.
If you get an error at this point, contact your local network tech and let him know what error message you received.
If the F: drive (or whatever drive is your network drive letter) is missing completely, you'll need to remap your drive letter.
Click Tools | Map Network Drive | click the down arrow at Drive and choose the same network drive letter that your icon was referencing in the Working Directory setting, i.e. F:
Click the Browse button and choose the correct computer and folder that this drive should be mapped to.
If you're unsure, go to a workstation that is working properly and double click My Computer. Look for the words before their Network drive letter, i.e. "Apps on Server F:" means you would need to double click on the Server computer and select the Apps folder under the Server computer.
Once the drive letter has been re-established, you should be able to get back into CMS without errors.