Do as Tapeworm says. If that doesn’t work, press the left button and try powering up. I doubt it will work, but if it shoud you will be in original firmware. If it does work, copy the original firmware to the root and repeat the procedure. This will reload the original firmware and all should work. There will be some left over Rockbox files in the root directory.
If you really want Rockbox, don’t give up. I installed according to the Rockbox instructions and everything worked fine. I removed it but might give it another try, but was just my choice, nothing to do with the way it worked.