Without really thinking, I used the "clear data" command on the Media Storage app, which deleted all the stock ringtones and notification sounds from my Nexus One. How can I get them back?
"all the stock ringtones and notification sounds" cannot be deleted without root. What happened here is, you've "dropped the index": the "Media Storage" app is what scans your media files from all over the device (including those on SD card), and maintains an "index" in form of a database. This is done for faster access, so apps using your media files have not all to scan the entire device themselves, but use a "cheaper and faster" call to that database instead.
What your "clear data" on this app did was not removing the media files themselves, but emptying that database – so apps relying on it won't see the files anymore. This gets fixed the next time the media-scanner runs. If you don't want to wait for that, the easiest way to force the media scanner to run is rebooting the device, as it always runs on "boot completed".
If you are rooted, you can push this collection of media files to /system/media/
Without root, android should still locate the files if you place them in a folder on your SD card called "media" and retain the rest of the file structure from the zip.
I did a restart of my phone and everything was back to normal. Hope this helps someone.
I have been constantly facing no-enough-space-remaining errors, which would block certain services and this had probably did some messing up with my ringtone settings. After restart, I was able to find my ringtones back where it should have been.