If you didn't create an export (OPML) then you may have to reimport the subscriptions from the preferences out of your timemachine backup.
In NetNewsWire 3, restore the application preferences within ~/Library/Application Support:
~/Library/Application Support/NetNewsWire/
In NetNewsWire 4, this changed. The preferences are now under ~/Library/Containers:
~/Library/Containers/com.blackpixel.netnewswire/
Procedure is the same:
- Quit the application
- In finder go to the folder (Menu > Go > Go to folder) and type ~/Library press enter an browse to the correct folder
- Activate Time Machine
- Restore
- Confirm to overwrite the existing folder
