IPhone 4s contacts randomly deleted?

I've had my iPhone 4s since December of 2011 and never had any issues with it. Last Monday, I was out with my friends and wanted to send a picture to one of them and realised that all my messages no longer had contact names. I then went into "contacts" and saw that absolutely every single contact was gone/deleted. I had done nothing. Half an hour beforehand they were all there.

They all just disappeared on their own. Does anyone know why this has happened? I do not sync my phone to a computer or iCloud because I have an extremely old laptop which tends to play up with anything related to Apple. I lost a bunch of phone numbers forever (not that it matters, I never keep in touch with those people anyway) but I just wanted to know why this happened. Is it a virus or what? Other people have said they experienced the same thing. Nobody knows why though.

My sis-in-law had same issue, and i did too a few months ago.i'm not sure what i did to get my contacts back, but i told my SIL to go to Settings -> iCloud -> Contacts. Make sure it is on. If it already is, turn it off then on again.hopefully this works for you because it just did 5 min ago for my SIL.