It depends on what level your issue is you want to send a comment about. If your stream is still running, you can simply prepare a scene with your message and switch to that scene to show your message. If all is well, you switch back to your actual content. If you need to reconfigure your scene setup while you show that message, use Studio mode of OBS.
However, if you're offline and cannot stream, it's a streaming provider specific issue to set up some "we're offline" message or image. You have to set that on the streaming provider, not within OBS.