Viber shows disconnected when you are on 3G, but connecting to a WiFi connection fixes this. If this is the case, then there’s probably an issue with using the internet on your phone over the 3G connection. Try to see if you can use the internet (try to view a website you’ve not been to before so it’s not cached).
If Viber is disconnected because of an internet issue on your phone, check aeroplane mode isn’t turned on and that data is set to “allowed”. Failing this, restart your phone.
If Viber still says Messages and calls are unavailable, it may be that Viber is disconnected in your location – your telephone provider or local government may have blocked the use of Viber in your area. Alternatively, Viber themselves may not provide the service in your area. Check with other mobile phone users if Viber works on their phones to ensure that Viber isn’t disconnected in your area.