External hard drive unplugged while running iTunes...Corrupted files
The first recommendation is to run Disk Utility to repair the external drive. If it is a simple corruption Disk Utility can occasionally fix it.

The next step from that would be to run a higher-end data tree repair utility, such as Disk Warrior. This software costs a bit of money but is better at repairing serious directory structure issues such as this.

You could also attempt a software-based data recovery using a program like Data Rescue. This software is just a bit pricey though, and at some point you will want to consider how valuable this data is. For instance, if it's a simple matter of re-downloading purchased movies or re-ripping DVDs it may be worth saving the money.