I had the unsettling experience this morning of my iMac not booting up properly. I knew I was heading in a dangerous direction yesterday when it reported to me that its disk was full. I know the cause. I have been putting a lot of media files into iCloud. I have two cloud systems which I believe is a mistake. Both are disk hogs.

I found this article and am following its suggestions.
https://www.howtogeek.com/217158/what-to-do-when-your-mac-won’t-start-up/