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I think you have accidentally changed the
Settings>General>Preferences>Layout to
hide the sidebar.
The sidebar is the column at the left of the Mail, Contacts, Files, and many Settings screens. There are two settings for display of the sidebar:
- Show sidebar: The sidebar is always shown at the left side of the screen.
- Hide sidebar: The sidebar is normally hidden. It can be temporarily shown by clicking a button with three horizontal bars in the upper left corner of the screen. This special button will only be visible if you are in the Hide sidebar setup mode. In the Mail screen this special button is shown directly to the left of the Archive button at the upper left.
If other settings (such as the Sort setting) are randomly getting reset, then something truly strange is going on. I would run a scan for viruses or malware and also try a different browser if possible. Of course, it's possible that something is broken at the Fastmail end, but I have not noticed anything unusual on various PC's and iOS devices I use many times every day with several different Fastmail accounts.
Are you using the normal Fastmail login or the beta.fastmail.com login?
Bill