Ezra 4

Then the people of the land tried to discourage the people of Judah. They troubled them in building, and hired counselors against them to frustrate their purpose all the days of Cyrus king of Persia, even until the reign of Darius king of Persia. vv.4-5

Read chapter 4
Facing opposition doesn't always mean a closed door.  As we see in Ezra just as the captives were returning to their land laying the foundation in the rebuilding of the temple they were stopped by their adversaries, but this was only for a time of about 15 years (which could easily feel like forever) till they were able to start building again and finishing it.  Enemies will do whatever they can to close a door that God has opened.  We see right here that these enemies of Judah first try to build the temple with them (v.2) but this was not truly to help but a trick to confound its building.  When they are denied to join in the building of the temple they then try other means to stop this project.  They tried discouraging them in numerous ways of troubling their plans until ultimately they resort to getting the king of Persia involved and trick him in to stopping the building of the temple.  We will see in the coming chapters that they realized this opposition was not from God but from man, so they press on to complete the work of God in rebuilding the temple.  
When have you been made a target for people to discourage and trouble you as you aim to serve the Lord?  How can realize if a "closed door" is from the enemies or from God?