"Your connection is not private" error message

Someone borrowed my computer and changed a few things, and now when I try to go to certain sites (like my Visa account) on Chrome I get the following message:

I'm using Chrome version 38.0.2125.111, and I can't access the site on Safari either. I have also had trouble loading images on certain sites sometimes. I'm not tech savvy at all, but I have done a bit of research and the certificate seems to be valid when I check the keychain access. My time and date are correct.

I'd be grateful for any advice, I'd rather not spend a bunch of money to take it in to the shop if it's something I can do myself.