In the %userprofile%+AppData\Local\Google\Chrome\User Data\Default
folder in Windows, there is a file called Bookmarks -> replace it with the Bookmarks.bak or use windows file history function.
(Backup bookmarks.bak before as well)
More info here:
How can I restore bookmarks after sync – Google Product Forums
https://superuser.com/questions/480670/undo-google-sync-in-chrome