Turn on the phone and then swipe from right to left and scroll down to the Photos application. Open the application and then swipe between the items. It's possible you've selected another tab that doesn't contain any images at all. Additionally, if the files were deleted, you can't recover them as there is no application for this operation. In case the pictures have been stored to a cloud, access OneDrive and you will have the possibility to access them.
If you reset your phone from About phone, you will be asked if you want to restore the backup to your phone.