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 you are following the series of articles in How do I troubleshoot crashes, errors, and performance issues in Fortnite on PC? and this did not solve the issue try the next step in our troubleshooting: What are the system requirements for Fortnite on PC and Mac?