The chances are that you aren't saving the right data in the DB, or you aren't retrieving it and saving it again properly.
The easiest way to save a file in SQL is to load it directly into a byte array, and save that:
byte[] data = File.readAllBytes(pathToFile);
And to write the bytes back to a file is also trivial:
File.WriteAllBytes(pathToFile, data);
There is info on how to save and retrieve it to the DB here:
Why do I get a "Parameter is not valid." exception when I read an image from my database?[
^] - it's based aroung images, but the principles are exactly the same for video data.
But...I wouldn't store video data in a database: it's big data and it will cause a lot of bottlenecks on your system to save and retrieve it.
Instead, store the video as a file and store the file location in the DB.
The way I'd do it is to copy the file to a "known" location with a Guid filename using File.Copy and then store the Guid-based path plus the original file name in the DB.
You can then recreate the original by using File.Copy to put it back when needed.