stewie - I did better than that. I decided to resort to that last ditch tactic called ..... RTFM!
That's short for "Read the Freakin' Manual", something that is hardly ever done!
So, it says on page 8, the impeller is self cleaning and doesn't need much care. Hmmm. Then why did I mess with it? See page 9 for impeller maintenance. Hmmm. Why didn't I read this before? I dunno, it all seemed so straightforward.
At the bottom of page 9: "specific notes concerning Fluval 304 and 404" ...."comes with a ceramic shaft fixed on the bottom of the impeller well by a rubber support. ...(reinsert the ceramic shaft) paying close atention to the rubber support to ensure that it (the shaft) has not come out of its seat."
So I am thinking, rubber support? I never saw any rubber support. I just stuck the shaft and impeller right back in never minding anything else. Well, guess what. When you make sure the ceramic shaft goes into the rubber support at the bottom of the impeller well, then the filter starts right back up after plugging it in.
Duh. I feel so dumb. But, on the bright side, never miss the opportunity to let others learn from your own stupidity! So, for all you fluval 304 and 404 owners out there, pay attention! Read this, and RTFM!