Was anyone inside the computer before this happened? I agree with the other posts. If someone was inside the computer, they may have bumped a cable or unseated a card on accident. If no one was in the system, then something might have blown.
Troubleshooting: Start simple by checking all the connections. Verify everything is in tight and where necessary unplug an reseat devices (HDD, vid card, memory, etc.) If this doesn't resolve it, start removing unnecessary components (these may be items that are necessary for work, but not necessary to boot the computer for testing, i.e. modems, hard drives, cdroms, floppies, etc.) If it will not work with the basics (PSU, CPU, MoBo, Memory, and vid card) start replacing parts one at a time and see if you get a post. I would start with PSU, Memory, and vid card and then work down to CPU and MoBo... You should be able to find the culprit by following these steps.
Let us know how it goes.
Bug