Don't panic, and reboot your phone. It's not a magical cure-all, but definitely a magical cure-almost-everyting. If that doesn't do the trick, check if you have enough calling credit or free mobile data remaining on your SIM card. You can do so on the website (under 'My products' > 'More info') or in the Viking App. Has your data been used up? Topping up would be the best way to go then.
If you have enough credit, you can check your settings to see if they are still correct. If that doesn't solve matters, contact our customer care team and we will check out the problem straight away.