hierarchical tree of Collections not visible

Hi, i use Zotero 3.07 on Ubuntu 16.04. In an older version i used to have three columns. The left one showed the hierarchical tree of Collections in My library, the middle one all files and the right one the single literature citation.
Now I can not see the left column any more.
When I now created a new collection or subcollection, I have no access to my data any more. So I need to have this left column visible to return to the main library. I would appreciate some help how to do it.
Thanks a lot.
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