Save as a draft
This is rediculous! I had to rekey my very detailed email 3 times because the system doesn't save drafts automatically ... and, there is no way to manually save a draft. Very poor design!
Please try clearing cookies, cache, history, and footprints. To do so, please go to https://help.yahoo.com/kb/new-mail-for-desktop/sln27197.html
Also, please make sure to update your browser and if the problem still persists try to access Yahoo Mail in another browser.