Hello @shuqing, thanks for reaching out to us here!
If the files that were backed up were synced to your Dropbox account, then the files should be present in your account. However, if the files added to the Dropbox folder was more than the space your Dropbox account can hold, then not all of the data will have been able to sync to your account.
Can you clarify for me what happened after enabling the computer backup for your desktop app? Did you delete the files from the Dropbox folder?
Also, if the data is in your account then they will be kept there until they're deleted or if your account becomes inactive. If you have deleted a file, then you can restore the files depending on your account's version history window.