Reboot and try again. Or, open the Task Manager (right-click on the task bar and select "Start Task Manager"), look for a process named "mshta.exe" on the Processes tab and kill it ("End Process" button), then start the TD again.
Every once in a blue moon when the TD exits, the window closes but the process doesn't actually exit. So basically the TD is still running without a visible window. Since only one instance of the TD can be running at any time, trying to start the TD only causes the currently running process to "come into focus", but since it has no window, nothing appears to happen. Once you get the original process to actually exit (by killing it or rebooting), things return to normal. Hopefully this is what has happened in your case.