PROBLEM:
When you try and Syncronize to Central, Close or Exit you get an error message saying Central File can not be found.
And Revit won't let you leave the file.
POSSIBLE CAUSE:
Some other method other than Revit Save As (e.g. Windows Explorer) has been used to rename the central file, or to move it to another folder.
SOLUTION - get out of Revit:
Press Alt Ctrl Delete, select Task Manager, select Revit from the Applications list, hit End Task button.
SOLUTION - create a usable Central File:
You can rename the file back to what it was, or move it back to where it was.
If this is not feasible you can create a new Central file.
Open the moved or renamed Central file.
Save As.
Click on Options button.
Tick Save as Central File.
Hit Save button .
(you can keep the same file name).
Close the file.
Go to Open dialog from within Revit via top left R symbol.
In the Open Dialog tick Make New Local.
HOW TO AVOID:
Don't EVER rename or move a central file from Explorer.