The path to the project that did not open is displayed near the top of the Unable to Open Project dialog box. If the project is:
on a network, make sure that the network connection is still active.
on a removable medium which is not properly inserted, insert it.
Then, if necessary, do the following:
Close all open FieldWorks programs (FDOBrowser, too, if installed).
See if FieldWorks.exe is still running in your operating system's Task Manager. If it is, stop it with End Task or End Process.
Open the project folder for the project. Delete the *.fwdata.lock file.
Reboot your computer.
The commands you see here match the commands you see in the Language Explorer dialog box.