There will be upto 10 ". bak" files named "downloads(date of backup).xml.bak", just rename the one with the latest date to "downloads.xml" while bitcomet is shut down. Shutdown your PC, after few minutes, restart your PC.
click on your bitcomet and all the task will be displayed.
For windows 10
1. Enable the show hidden files, folders or drives
2. Locate downloads.xml.yyyymmdd in local disk (C:)>user>username>AppData>Roaming>BitComet>
3. Rename downloads.xml.yyyymmdd as downloads.xml
4. Shutdown the PC
5. Restart PC
6. Open Bitcomet and all tasks will be listed.
This worked for me.