Hi
All the content I made in K2 - except the home page - has disappeared since yesterday. The menus are still there, but blank content.
www.swimhistory.org

There are more than 500 K2 articles, created over 10 years, so please advise how I might recover it.

Hello,

Try repairing your database's tables. Are the items present in the adminstrator area?

Hi. All my content created since Feb 2017 has also disappeared. I run K2 v2.5.7 and Joomla! 2.5.6

Both Joomla! and K2 are severely outdated.
This poses a real security issue, furthermore try contacting your hosting provider. Maybe an older backup was reinstated.