Names not shown on WhatsApp anymore; contact details suddenly gone

My phone is Samsung Galaxy Note 2. All of a sudden, halfway using WhatsApp, the names are not shown anymore. Instead, I can only see the number. I went to my contact list and checked. Most of my contacts appeared to be shown in email addresses. And it's the same person's email address. I clicked in and found that the detail of these contacts is gone. What happened to my phone and what should I do?