This morning I faced a strange issue of watching my WhatsApp live tile grayed out. I tried restarting the phone, and hard reset (volume rocker key (-) + unlock key) to no avail. Little scouting on support forums provided the answer:
When Windows Phone 8.1 tries to backup data to cloud, it disables the app communication temporarily, so that backup is consistent. This stage should be over in 2 minutes maximum. However, if the backup process is stuck due to connectivity or other issues, it takes time, or never sends signal of resuming communication for the app, WhatsApp in this case. This renders the WA inactive, with tile grayed out.
Go to Settings->Backup->app+settings-back up now.
Once the backup starts, wait for few minutes to complete, if it does not, cancel it explicitly. “Cancel” action will intimate WA to resume communication, and your tile will be live again.