This situation occurs when your Revit session can not find the central file. This could happen through no fault of your own. For example it could happen because the project's server is down, a router is malfunctioning, your network cable was unplugged by the cleaning crew and so on. Of course those would present themselves in other more obvious ways like not being able to get your email, see folders on the server or even log onto the network. It could also be caused by someone deleting the central file, the central file's backup folder, someone saving over the central file or some other nefarious circumstance.
It can also be caused by taking your local file home with you. When you open a Revit project, that has had worksets enabled, Revit "talks" with the central file. If it can't find it you'll get an error message.
That's your first warning that things aren't safe to continue working. At this point I encourage you to stop, see if you can find out what's wrong, assuming you don't already know. If you push forward and try to alter something in the file, using element borrowing, you'll get this message.
That message is preventing you from using element borrowing. If you really want to continue to work at risk you'll need to open the Workset dialog and make the workset the element is assigned to editable, which makes you the Owner of the workset. You won't be able to do this either without getting another warning dialog.
If you click Yes you'll be able to do what you want with any elements assigned to the workset you've made editable (you are the owner of the workset). This is where things will go very wrong. If someone else does this too or still has a valid connection to the central file you are working "at risk". As soon as another person does something that Revit has either given them permission to do or creates a conflict of ownership somehow...the first person to resolve it will win...the rest will lose.
If you must go down this road you need to discuss what you need to do with the other people working on the project so they either stay away long enough for you to get things done or agree to be very careful about what they do. Go slowly, methodically...carefully.
Other Workset Topics to Read