Open a command prompt. Change to the eclipse home directory and type "eclipse -clean" e.g. C:/eclipse>eclipse -clean
This will ask for the workspace selection. It will also force to set it as the default workspace.
Then, go to eclipsehome-->configuration-->settings folder.
open org.eclipse.ui.de.prefs in a notepad.
set this property to true from false.
SHOW_WORKSPACE_SELECTION_DIALOG=true
You will be asked for a workspace selection everytime.