I GOT THE ANSWER!
I've just made an account just to tell you guys! I was puzzled by the same problem, but I figured it out!
Go to your photo album and try to share a random photo via Whatsapp.
Now the app "Whatsapp" is being openend from this perspective.
First I saw a black screen with the word "Whatsapp" in the upper left corner, but then it said in the middle of the screen and using the Windows Phone dots which move from left to right: "Recovering".
I've tried this once and I had hope, but then nothing happend. So I tried it a second time and it worked(!), the app opened and all my messages that I did not recieve in the past 24 hours, due to this weird problem you all describe, are all there and marked as unread!
Plus the tile has it's normal colour again!
C'mon!
Greetings from a Windows Phone enthusiast from the Netherlands!
Alright, I have to admit, there are a few things that could be worth noticing!
1. My chats are in the right order, but the dates are incorrect for some of them. Some people who have contected me, lets say, three days ago, are listed as a date in october. But when you open the chat, the dates and times are correct. I already found out this can be fixed by just talking to those people so they move up in your list and the date will automatic adjust to the time you just send a message!
2. I have found out that I used Whatsapp exactly one year on the day my tile went grey (7th of november)! But now, when I go to my account, it says I can use Whatsapp until 7th november 2015! Some of you suggested that it had nothing to do with the date when u used Whatsapp exactly one year, but I am pretty sure I used it for the first time in November 2013, coincidence?
3. The final thing I've noticed is that when I open Whatsapp, I do not see the green Whatsapp background anymore for a short while. Is that common after the last update or do I now have an unstable Whatsapp application? If so, the solution is to archive all my chats (which I didn't lose) and reinstall the app. When I do, I will report back!