pictures are darker than should be

Hello, Windows 7 Home, JPEG images, opens with Windows Live Photo Gallery. In my Pictures Library, when looking at the groups of pictures, they look normal. When I pick any one of the pictures and click on it to open it, the picture looks darker than normal, and it prints out darker than normal. I haven't downloaded any new programs relating to Photo editing or anything like that. Any ideas on how to correct this (keep in mind please I'm older generation and not real computer savvy, thank you.).

No, I didn't check those -- when I open the folder to see all the pictures in a folder, everything looks fine and normal. It's when I pull up any one of the individual pictures and also it's when I print it that they appear dark.

I figured out how to adjust the settings for each individual picture, but all the pictures seem to do it. Is there a reason they all got dark, and maybe an adjustment for all the pictures, or does each individual picture have to be manipulated one at a time? Thanks for the help.