why can't I view my photos on computer-Help!

[Mimmit]

I am in the middle of ebaying loads of stuff but for some reason my photo files have changed. Instead of being able to scroll through and see them to upload they all come up with those picture symbols (hills and sky iykwim)
I don't know what to do. I am really rubbish with computers. A while ago someone put my photos on picasso (?) (the google thing) would this have anything to do with it.
To make it worse my camera and memory cards were stolen a couple pf months ago so I really want to make sure my photos are safe and that I don't delete them by mistake. Can any of you experts help please?

Hi science, thanks for replying. It is Windows 7. I am going through them now and can see they haven't been deleted, I can go through some on slide show but i think I have changed their format somehow. A lot are JPEG and some are MPG4. I have found something about default programs. May that help?

Hi again. Sorry not to reply. DS woke up and that was that. I can open both. With MPG4 I have to click open option then use default program.
I have since found all my pics on picasa 3 so I now just need to work out if I can upload them from there to ebay.You may guess I am clueless!
All this has made me realise I need to organise my computer files and declutter as I am doing with the house. Some photos appear about 6 times in different places. Another job to add to the list.
Thanks again for your help.