all my bookmarks gone! for no reason, they were safe for weeks until this morning

I had them all set up with a kind of good logic, stuff that I do not use every day and other bookmarks.

It will take ages to find all the websites again.

Why on earth were they all wiped out? Yesterday it was working fine, as it had for weeks. This is not 'reading list' - that is still untouched.

Bookmarks are all gone, except a folder which has only one of the addresses I was using the rest is gone.

I cannot find a way to retrieve them.