From Dell's trouble shooting guide:
#If the power light is blinking amber, the computer is receiving electrical power, but an internal power problem might exist.
* Ensure that the voltage selection switch is set to match the AC power at your location (if applicable).
# If the power light is steady amber, a device might be malfunctioning or incorrectly installed.
* Remove and then reinstall the memory modules.
* Remove and then reinstall any cards.
* Remove and then reinstall the graphics card, if applicable.
* Ensure that the processor power cable is securely connected to the system board.
# Eliminate interference. Some possible causes of interference are:
* Power, keyboard, and mouse extension cables
* Too many devices on a power strip
* Multiple power strips connected to the same electrical outlet
#If the power light is blinking amber, the computer is receiving electrical power, but an internal power problem might exist.
* Ensure that the voltage selection switch is set to match the AC power at your location (if applicable).
# If the power light is steady amber, a device might be malfunctioning or incorrectly installed.
* Remove and then reinstall the memory modules.
* Remove and then reinstall any cards.
* Remove and then reinstall the graphics card, if applicable.
* Ensure that the processor power cable is securely connected to the system board.
# Eliminate interference. Some possible causes of interference are:
* Power, keyboard, and mouse extension cables
* Too many devices on a power strip
* Multiple power strips connected to the same electrical outlet