If you are getting a specific error message in Fortnite, copy the exact error message into the search bar that reads, "Search Help Center..." This should bring up an article for that error message.
You can also visit the Fortnite Community Issues website. This has a list of known issues. Some of these issues will have workarounds that can get you back in the game.
If this did not help solve the issue try: What are the system requirements for Fortnite on PC and Mac?