In case you find yourself in Greece without a quest to complete, it´s necessary to check the city of Athens or the Olympus.
When playing the pc version of the game, please go to Map Manager and you´ll see the Athens instantly.

In case you didn´t find the new quest in the City, it´s necessary to look in Olympus. Please click on the Main tab and you´ll be able to access the Olympus from there.

The interface for the app version is a bit different, however, the idea is the same. In case you´re not able to continue in the game and you need more quests, first, check the City by tapping on the city icon:

In case you won´t find a new quest in the city, please go to Map Manager, tab Greece and check the Olympus. It´s located among the locations of Zeus, therefore, in case you´re further in the game, you might need to swipe to the right a bit. The icon of Olympus is different than the others so it should be easy to recognize it.

Hopefully, this will help you guys 🙂 Good luck!
In case you´re having issues finding a new quest in Egypt, please check this article.
If you´re having issues finding new quests in Scandinavia, please see this.
In order to find more quests in China, please look for a piece of advice here.
To be able to get more quests in Atlantis, make sure to visit this link.
And if you´re in need of more quests in the New World, please check the steps here.