One diagnostic is activity on the rabbit tip window. Generally this window shows almost no output except during reboots and during DSP reboots. To recover from such a crash, it is usually enough to select Reset NIRC Software from the pull-down menu. This will automatically kill and restart various components of the software. It will then run through the same software that is run when the software is first started, so you will again be asked for your data directory.
Some crashes are more insidious.