I went into Settings->Privacy->Factory reset and checked "Format USB Storage". Then I pressed the button to continue and it prompted for my PIN (just numbers) as usual. Then it said it was formatting the USB storage as I told it to. And then it just shut off. So I started it back up and it prompted for a password (full keyboard; any characters you want).

I don't know what I had set that password to (I haven't used that kind of a password in a while) so now I'm stuck. Going into recovery mode does the same thing. Download mode works just fine.

So now what do I do? Is there some way I can format `/data` manually from my computer?

Here's some more info about my device:  

Samsung Galaxy Player 4.0  

Samsung Galaxy S build.prop  

Rooted  

Stock Gingerbread ROM, stock kernel, stock recovery