IntelliJ IDEA Educational Edition

I'm learning Java at JetBrains academy and I'm doing my first project called Cinema Room Manager. Today I made a mistake in IntelliJ I have accidentally right-clicked on my Problems folder and choose Mark Directory as > excluded and I can't see it anymore. Moreover, I can't continue study because new Problems that I'm trying to solve not compile.

PS: Problems was a folder "if it correct term" that I solved during the studying process it was on the left screen under my Cinema Room Manager project.  I'm completely confused and don't know what to do next.