So, you have a video DVD that definitely has data on it (you can see the lines on the shiny side), but that looks blank when you insert it into your computer. Don't worry, it's not ruined. The data really is there. What has likely happened is that whoever burned the DVD for you failed to "finalize" the disc before giving it to you, so the player/operating system has no idea what to do with the data that it sees. Fear not, though, there are options.
First, we're going to get some software. I'm assuming that we're on a Windows machine here, but these utilities all exist for non-Windows users and I'll leave the minor translations there as an exercise to the reader.
Go to this page to get pre-compiled Windows versions of the dvd+rw-tools programs. The one that you need is called dvd+rw-mediainfo. We'll be using this to read the structure of the disc to figure out where our data lives.