Hello there, Mnaoom3.
The following Knowledge Base article offers up some great steps for troubleshooting an App purchased from the App Store:
Thanks for reaching out to Apple Support Communities.
The issue you are experiencing may be due to internet connection issues.
Please make sure you have a strong Wi-Fi.
For further information, please follow the link: http://bit.ly/19QCPOf
Have a great day!
Viber Support Team
Hi Viber Team,
Having problems syncing contacts in Viber on my iPhone 4S. This happened after the latest update from Viber. It has been syncing contacts for hours but shows no contacts either in the All list or the Viber list. Have deleted and reinstalled the app and have also tried switching off the phone and putting it on. Please help.