Deleted categories

I have a huge problem. I have deleted all categories on my emails. I opened the folder “all emails” and changed the categories on all my emails to “no”. Now, I cannot find any emails in my different folders, only in the folder “all emails”. I have many thousand emails. Is it possible to UNDO this operation. I haven’t backup…

This is the way Gmail works. All the messages only exist in one folder called All Mail. They are then categorized into virtual folders like Inbox, Sent etc. by labels (categories). So if you delete their categories, they will not be sorted into those folders.