Put your iPod into recovery mode: http://support.apple.com/kb/HT1363
Start iTunes.
Plug your iPod in and let iTunes completely wipe and reset it.
Go through the Rockbox installation procedure again.
...
Please don't do that.
It is absolutely and completely unnecessary. I appreciate the intention, but the first step of problem solving shouldn't ever be formatting the device.
cecobask, two* options open to you:
1 - Boot into the embedded Rockbox fallback image via:
emCORE main menu -> Tools -> Run Rockbox fallback image
(if you have emCORE's fastboot mode enabled, you will have to press and hold any key during boot to expose the emCORE main menu)
If you can successfully mount the device from this fallback image, extract a
current Rockbox binary to the root of the device.
2 - Run UMSboot from emCORE via:
emCORE main menu -> Tools -> Run UMSboot
Now you can extract
a current emCORE installer binary to the UMSboot volume. Because emCORE is already installed, you should not lose any of your data as the disk will not need to be formatted. Procede with the installation, and then shut down the device and reboot into the emCORE main menu (as described above), boot the embedded fallback image (as described above), and extract a current Rockbox binary to the root of the device (again, as described above).
If your host machine cannot mount the embedded fallback image in either case, you will not be able to use Rockbox without an emCORE binary that exposes its own form of "Disk Mode" from us over at
#freemyipod-support, the Freemyipod project's IRC support channel.
[Saint]
*(well, technically three options if you consider restoring the device with iTunes and re-installing emCORE an option)