Just take it apart and put it back together... Fixed... lol
A lot of the newer Dell models have a pretty handy feature on the motherboard. On the rear I/O plate there are (4 Diagnostic lights).
After a sucessful post, those four light will be solid green. If your dell is failing it's post, there will be specific light patterns which are outlined in a per/model online .pdf or in your user manual.
Here is a .pdf for the 4700 Series.
If you would have known this info, you could have possibly used those light codes to find the problem.