I regularly back up my DVD collection to an external hard drive
because I travel alot and like to be able to watch what I want without
the choppiness associated with reading straight from the drive.  I
copy the DVDs to the hard drive as ISO images and then load them using
Nero's ImageDrive software, and then play them either with Interactual
Player or VLC.  This has worked for the past couple months.  However,
just recently, I tried to play one and no video came up.  I tried
another one and again, no video.  It seems to be consistent throughout
all my collection.  I have tried re-making the ISOs and it doesn't
bring back the video.  What may be causing this and/or how can I fix
it?

Normally it happens when there is a problem with the codec try to install
k-lite mega codec pack.i hope it work if it doesn't the problem could
have risen due to problem with nero image drive try using alchol 120%
it's the best virtual drive you can get its fast and works great.