Unplug the power cord, open the case, then temporarily disconnect all the "non-essentials". Disconnect the external "non-essentials" as well. In other words, all you need externally is the monitor & keyboard. Internally, disconnect ALL drives (floppy, CD/DVD, hard drive), remove all RAM except for a single stick, & remove all cards except for video. While you have the case open, clear out any dirt/dust that's accumulated, then reset the BIOS using the clear_CMOS jumper.
Once you've done all that, plug in the power cord, power up the system & see what happens. If you get a display, use the keyboard to enter the BIOS & correct all the BIOS settings.