I reinstall my windows and bitcomet. And now Bitcomet doesn’t recognize my unfinished downloading files. I still have the .bc! file. How can I resume the downloads?
Thank you!! 
Copy the partial files into your download folder, then start the torrent. It should then check (rehash) the files and continue the download. If it doesn’t, stop the torrent, and do a manual rehash (right click, manual rehash).
If the files are in the correct location, this will work.
Suspect
ps. be sure you have the .bc! added to partial files selected in your now install, if your partial files have that extension.
I don’t have the .torrent file (only have the bc!). I have to search for them right?
Your Torrents are automatically saved in \program files\bitcomet\Torrents in case you ask your OS to open the torrents with BitComet. When you have chosen to save the torrents yourself to open them later, they should be on the location where you wanted the OS to save them.
If you’ve opened the torrents with BitComet the torrents are gone because of your reinstallation.
In either case, when you find them, do as Suspect wrote.
Petch01
This is why you should save .torrents to a folder on your HD and then open them.