Check Google+, they may have been uploaded to the Google+ cloud backup. That shouldn't have made them disappear, but I've been seeing lots of similar questions lately.

 

Welcome to the forums, Katie. :hello:
First, are you just looking in the Gallery or the Photos app? If the latter, your photos are (hopefully) backed up via G+ and should be able to find them there. They're not stored on your phone, so you'll need an internet connection to get to them. Another option is to look them up on your internet browser (on a PC for example). Google "google plus photos" and just log into your Google account.
But, if they're not backed up, I'm really not sure how else to bring em back.