Your diag firmware may be pooched. During bootup do a CTRL+C. That will get you into the pre-loader (not the BIOS). From there you can Z-Modem the firmware back onto your switch. If you are in BIOS, that may or may not work. There is a great doc on the Avaya web site that will show you how to do this, just search for Z-modem & your switch (ie. 4500).
Cheers, Uncle John.