Hi - if by pressing F12 during the start up phase you can select a boot device, I'd suggest trying to boot from another device, such as a USB stick or external floppy and testing the HDD. My D255 would attempt to boot but halt at a blank screen with a flashing cursor in the top LH corner and with the HDD light on but dimmer than normal.
Most HDD manufacturers' HDD test utilities won't work with the Toshiba MK256GSX drive but I had an old floppy with an Hitachi HDD test utility on it. That worked fine - in my case, the HDD tested OK and it looks like I must have overwritten the boot sector (Ironically, I'd just completed a Ping backup and the machine wouldn't boot up again). I'm going to try an earlier Ping image that I made just after I bought the machine and see if that works. Good luck!