Blank video duration 0:00

I recently upgraded to a faster server. Since then I have noticed several videos uploading with 0:00 duration and will not play. I do not believe it is a problem with the videos themselves as the 0:00 duration has shown for videos uploaded by different users and only began after I recently upgraded to a faster server. How can i;

1. Fix the existing videos with duration 0:00 so they function normally on the site.
2. Prevent future videos experiencing this problem?

Many thanks

Make sure that all of the required software is installed and that your configuration file is pointing to their correct location. Once you do that, you should be able to rerun the convert process, which would fix any of those errors.

Thank you for this advice.