Please help me understand why I can't find my videos

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Hi,
This is my first post. I take a lot of pictures and videos with my Canon Powershot A540. However, I'm not very good with computers. A few months ago, my computer crashed. The repairman was able to recover a fair amount of my files. Most of the pictures and some of the videos came back fine, but I would get a message saying that the player would be unable to play other videos when I clicked on them. I tried playing them in Quicktime, avi, and Realplayer, but none of the worked. Other videos just show up as THM files, which I know I can't play. Does anybody know what happened to the original videos? How come I only get the THM's and not the video itself? What happened here?!!! If you don't know any of this, could you give me some advice on who to talk to? Thank you