IntelliJ bad plugin how to start

If you look in the directory where you installed IntelliJ, you'll see a /plugins directory. Perhaps if you delete the Mercurial plugin folder you'll be able to restart the UI. Worth a try. NOTE: Check here for OS/version related plugin path.

Version 11 for Windows stores plugins also in c:\Users\<username>\.IntelliJIdea11\config\plugins\ so delete plugins from that location as suggested by @duffymo.

I am running on a Mac, the Plugin directory is located in ~/Library/Application Support/IntelliJIDEA60/ You can just delete the offending plugin. Note, depending on your edition of the IDE, the directory may be different. For example, IdeaIC2016.3