How to undo Gnus killing multiple groups

[Göktuğ Kayaalp]

This is a ‘cross-post’ of this reddit post: 
https://www.reddit.com/r/emacs/comments/8onz01/how_to_undo_gnus_killing_multiple_groups/

I hit some random key mistakenly while browsing my mail and news in
Gnus, in the group buffer. All my groups disappeared, not showing up
with L or l, but only with C-c C-l, and I used u to resubscribe to
them. I had years of marks and messages staying unread for months
waiting to be read, they are just gone. What do I do now?

I guess what I did was C-w, but C-y did nothing, neither calling it with
a numeric argument to yank multiple.  But I pressed R in between in the
hope that I could revive my groups as such.

Any help appreciated.  I need to find what messages were unread in one
group (very important, otherwise I'll have to go in manually), and my
important marks in various gmane nntp groups (which is not as important,
but still, they were topics I wanted to follow).

Thanks in advance,

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İ. Göktuğ Kayaalp       <https://www.gkayaalp.com/>
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