EDIT: No you don't have to detect a DFU, it just needs to see the iPod disk drive, "iPOD CLASSIC" or something in My Computer. Rockbox'd Ipod Classic doesn't like Windows 8 so if you upgraded to it or the later you might not be able to get it working. Find a computer with Windows 7, it is your temporary solution. you'll factory reset the iPod there, and you will be able to sync your songs again at home with your computer.
Here's my humble guide hope it helps.
If you have the latest Rockbox it will not work probably because... the latest 2014 version - for me at least - makes the iPod invisible on computer when plugged in, on Windows 7 64-bit. Hold the Menu and select buttons until you go to the emCORE boot menu. Select "Tools" , click "Run Rockbox fallback image" and copy the Settings, and download the "rockbox-ipodclassic-r31516-20120102" and extract the archive, so you copy the ".rockbox" archive's folder files to your ipod main directory, overwriting it. Then restart, run Rockbox. Now here's the part where it should work. Open iTunes , make sure your rockboxed iPod is on and connected to USB, so connect your iPod through USB to your computer, WAIT until it connects, it could be the reason for not working, THEN hold menu and select for long, so even when the emCORE menu and/or Apple logo shows up, keep holding it until you hear a click and see a black screen. Then release it immediately as soon as you see it. iTunes should detect the iPod and say 0 kb or something, don't worry, it says it needs to be recovered, let it recover, download the fixing tool etc. wait. Then it should be again Apple firmware. I rockboxed my iPod Classic 160gb 7g 3 times so i hope this works for you.
If iTunes doesn't detect it,
You have to enable the iTunes Apple Mobile etc. services in Windows. Go to Start->Control panel->Administrative tools->Services-> and enable as "Automatic" services: Bonjour, Apple Mobile Service, iPod Service, right-click and turn them on, restart the computer. Then run iTunes FIRST (important!), THEN connect your iPod through USB. Do the reset as told and hope it works.
sorry for crappy English, hope it helps!