Can a software "hard freeze" or serious hardware problem completely disable the whole cockpit?
If the power suddenly disappears to any aircraft (ie, a total electrical failure including the batteries), then all the aircraft systems will be failed regardless of whether or not they were glass.  This is highly unlikely because there are several power generation backups that would have to fail, including the batteries themselves, before this ever became a reality.  Unfortunately though, if the aircraft only has electrically controlled, hydraulically actuated control surfaces (fly-by-wire), vice actual mechanical linkages (or backup controls), then a total electrical failure would be 100% fatal if power cannot be restored.  The control surfaces would not be movable.

However, more to the question regarding software bugs, all primary flight instruments are backed up to prevent such disasters from happening.  I have personally lost all the digital displays in my cockpit and flown a no-gyro, surveillance approach in bad weather using nothing but a peanut gyro.  Its possible, but definitely not a good time.

Anything is possible.  However, even though the odds of a total electrical failure, or complete loss of instrumentation (including backups) is nominally greater than 0%, the statistical probably can still probably be rounded to 0.