PC boot problem

Dell Dimension 4700. Powers up, green light on mainboard, CPU fan runs, moniotor displays the graphic card details on the moniotr in the top left hand corner. No beeps at and it just sits there.

Swapped PCI express card for PCI graphics card. Same outcome.
Removed and swapped ram around. No difference.
Taken out all other add on cards.
Tried different hard drive.
Changed mainboard battery.

Diagnostic lights are all green expect for the 4th one which is amber.Manual says it is is trying to boot from a faulty or non existant boot device.

However I cannot even get into the bios.

Any advice please.