To see your pictures again in the gallery, just remove the .nomedia file, it's hiding the pictures in the folder from the media scan.
You need to trigger a rescan to be executed by the media-scanner (e.g. by performing a reboot) for your photos to show up. Only thing I wonder: new photos are shown, just old ones are gone – that sounds like a different issue, unless the new ones use a different folder.
New section contributed by Izzy