The flashing question-mark indicates that the computer is unable to locate a valid system folder.
You could try to
reset the PRAM to begin with. Having the voltage of the internal battery checked could also be a good idea.
The Disk Tools PPC (
here) should work as a startup disk for your
Power Macintosh 5500/225. Please note that another Mac (with a floppy drive) and
Disk Copy 6.3.3 (the Make a Floppy command in the Utilities menu) will be needed in order to produce a properly sector-copied floppy disk.
If necessary, it is possible to remove the hard drive and place it in an appropriate external (preferably FireWire, or perhaps USB 2.0) enclosure. This would allow access to the hard drive from a modern Mac.
Jan