RE: Dell XPS 8700 does not post after BIOS upgrade
Ron gave the best answer for self recovery, however the motherboard is not necessarily shot because the BIOS may be corrupt. Granted, buying a new motherboard is somewhat easier but more expensive.
Clearing the CMOS instruction set will permit the system BIOS to rewrite it. If the system BIOS is the wrong one, then it will re-write the wrong information again.
If no joy try this:
The fix is to purchase a new BIOS chip from a reputable retailer for around $17-$25 USD. Place the new chip squarely over the old one on your motherboard aligning each leg, tape it if you need to.
Start the computer.
Remove the good taped BIOS chip
The computer does not need the chip once it is started.
Flash the old BIOS with the CORRECT firmware from Dell.
I have done this more times than I can count to systems that were improperly flashed.