My phone contacts keep appearing and disappearing again!

I have an iPhone 5s currently running on IOS 8.2. Up until 1.30/2pm yesterday it was working completely fine until I tried to add a contact from a group message (which I has been part of for a while now and have never had problems with before) when all the other contacts in the group suddenly stopped showing their names and just showed the numbers. When I went on to the contacts half of them had disappeared, as had all the recent contacts and favourite contacts from my home screen. All my phone messages that used to show the senders names just showed their numbers so I had no idea which messages were from who. After a while they eventually all came back but it proceeded to do this again as soon as I started using it again with them constantly disappearing and reappearing. I tried plugging it in which helped when I first tried but not the second time later on and I tried turning it off and on but the contacts still didn't reappear. What is wrong?? Please help!