The problem was the RSS feed block. When I made the RSS feed block invisible, the blocks on the right hand side reappeared.
I guess that
This problem may be the same as that reported elsewhere by Crafton Williams and Paul Shephard.
The problem occured during the night while I was not doing anything to my blocks so I guess that the change (the disappearance) occured due to the arrival of something in the RSS feeds. Perhaps a title was too long, or contained strange characters? I guess that this may suggest the need to incoming feeds?
This was the problem
The blocks on the right hand side of my course pages had disappeared.
- The right hand side is there on the main site page.
- When I move blocks from the left to the right hand side they dissappear too.
- The add blocks block is on the right, so I can't add blocks.
- Pressingly, I would like to use Brian Koontz's wonderful Course Date Manager Block but that was on the right hand side so it has disappeared.