Hi,
If the power light is blinking amber —
The computer is receiving electrical power, but an internal power problem might exist.
Ensure that the processor power cable is securely connected to the system board.
Here is a link to the Quick Reference Guide for your PC. Scroll to pages 36-38 in the guide which explains what the diagnostic lights showing on the back of the PC mean. It may help you to resolve your problem.
http://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-gx620_reference%20guide_en-us.pdf