Half of my contacts disappeared on my iPhone 5

More than half of my contacts (app. 500) on my iPhone 5 have disappeared without any specific reason. Trying to add them again, deletes the contacts again within a few seconds. I first noticed in Whatsapp I could not add a mobile number through "Add" and "Create new contact". After creating the contact, it disappeared again within seconds. Checking my contacts list I noticed more and more contacts were getting deleted. Creating contacts through WhatsApp or "Phone/Contacts" results in the same and contacts are deleted again. I do not know whether WhatsApp is related to the problem.

Can someone from Apple please update me on this problem. It is blocking my work

iPhone 5

IOS version: 8.2 (12D508)

Whatsapp version: 2.11.16