It might be possible that the Designer fails to open a mapping or process after a machine crash.
You will have this error message in the main window, when trying to open the mapping :
There are several manipulations to try in order to solve this issue.
Solution 1 : Delete the file <your_workspace_dir>\.metadata\.plugins\org.eclipse.e4.workbench\workbench.xmi (note: deleting this file will reset some layout settings to defaults, such as the Perspectives and Views).
Solution 2 : Right-click on the mapping name and select Advanced / Reinit diagram.