Since you got the insert floppy disk screen when you started, the Miggy could not initially see the hard disk. Now you have reconnected it, the Miggy doesn't work.
I'd go for disconnecting the hard drive to see if it will boot up without it (Could be faulty hard drive OR power supply unable to feed all loads). Can you borrow another power supply from anyone ? An A500 psu is slightly higher rated and less likely to fail.
Since you don't know the history of the machine, it will be a matter of trying things one by one. :-D
