Dailymotion recently started purging old inactive content, probably to save costs. Unfortunately they don't have the kind of financial backing that YouTube has. This is a reminder to save the content if you value it.

From the terms of use:

You acknowledge that if Your Dailymotion Account remains inactive for a significant period of time, Dailymotion reserves the right to delete, reclaim or remove Your Dailymotion Account in its sole discretion with or without prior notice to You.