My PrntScreen key no longer works with Greenshot.... why?

Most probably, this is due to other software reserving the PrntScreen key. This could basically be any software that considers itself good for handling screenshots, but in most of the cases we hear of, the problem is caused by Microsoft OneDrive or Dropbox.

Are you using OneDrive? Look for "Auto save" in OneDrive's settings dialog and uncheck the "Automatically save screenshot..." option and you should be fine.

Are you using Dropbox? Several users reported similar problems after installing Dropbox or updating it. Newer versions of Dropbox come with a feature that copies screenshots to your Dropbox account. For this reasons, it registers the PrntScreen key and Greenshot is no longer notified when you hit the key.

Many people are confused by this, so obviously Dropbox does not make its changes to the system configuration transparent enough. Feel free to contact Dropbox support about this.

There are various workarounds available, pick one :)