Optiplex GX270 inert / yellow light?

Hi,

Just tried to reboot my old Optiplex for some urgent software/hardware tests then noticed an orange pilot light hadn't come on at the back. Ominous. Round the front the start button was flashing yellow. No signs of life otherwise, no bleeps, nothing. Last time I used this pc there were no worries at all.

Any ideas please?

Thanks