I delete the most important collection in my library by mistake, and it have synced with online library. How can I regain this collection and its subcollections?
note that the items in your collection are not lost - just the collection as an ordering element. There is no direct way to undo that in Zotero but you can restore the library from your last back-up
http://www.zotero.org/support/zotero_data#restoring_your_zotero_library_from_a_backup
or see if restoring from an automatic back-up makes sense:
http://www.zotero.org/support/zotero_data#restoring_from_the_last_automatic_backup
In both cases you want to make sure to first disable syncing and then "Reset to Server" from the Sync--> Reset tab of the Zotero preferences before re-enabling automatic syncing.
Upgrade Storage
http://www.zotero.org/support/zotero_data#restoring_your_zotero_library_from_a_backup
or see if restoring from an automatic back-up makes sense:
http://www.zotero.org/support/zotero_data#restoring_from_the_last_automatic_backup
In both cases you want to make sure to first disable syncing and then "Reset to Server" from the Sync--> Reset tab of the Zotero preferences before re-enabling automatic syncing.