Like I said, putting it into disk mode by holding menu+select and then flipping the hold switch deletes the rockbox partition.
You didn't mention disk mode, and I assume you meant holding play+select (that's what puts it in disk mode). Menu+select would reboot it or put it into DFU mode. Though I suppose if you rebooted it and then put hold on, you'd get into original firmware mode, so I'm guessing that's what you did?
What type of iPod is it? If it's a 6th gen and it's been modified to be >128gb, that could cause that problem.