Have you tried a soft reboot after the A1200 has been on for a little while? It may be that the hard drive does not spin up quickly enough, so the Amiga thinks it has nowhere to boot from. Try letting the computer be on for, say, 30 seconds and then do a ctrl-amiga-amiga reset.
Failing that, you can set up the hard drive (or a new hard drive) again with the help of the UAE emulator so you won't need to find any floppies necessarily.