Seeking any help on where to start trouble-shooting an HP color laserjet 3800 printer.

On startup, the printer cycles through it's routine checking itself, then stops with the display lit and all three indicator lights left on (Ready, Data, Attention).

No manner of button pressing gives any responses.

Where do I start troubleshooting?

I have the manual, but no guides to this type of problem.

Have you checked that the formatter is seated properly ?
Many weird display issues are seen on a "loose" formatter.
While the printer is turned off try to pull out the formatter by the two black handles and reseat it again properly by listening to the "clicks" when the formatterlocks gets engaged.
Also perform an engine test to verify the rest of the printer is OK. Use a small, pointed object to
depress the test-page switch on the back of the printer (located just below the small removable plate covering the the formatter expansion slot).