Putting iPod into Disk Mode. This is a very confusing mode and lots of folks seem to get stuck in. Basically, if you run a reset and then press Select and Play right afterwards, you get into a low level mode called (badly IMHO) disk mode. What this means is that only the very low level loader is running since presumably your iPod firmware is messed up. You then connect it to your PC/Mac and run iPod Updater (you get that from Apple.com). You can then "restore" your iPod which is another bad term IMHO, that means it wipes the whole iPod and you start over with a blank iPod (all your music is deleted) but a fresh copy of the firmware.
BTW, if you get into this accidentally, just reset the iPod again.