Just as a quick troubleshoot, without getting into the heavy stuff (mainboard, data ribbons, etc.), with the power off, check for loose wiring, or missing switches among the voltage selection switches inside and on the underside, to the left of the platen.
Had this happen to me once, and when I checked out the voltage selectors, the problem made itself apparent; the guy I replaced had broken off one of the connections, busting the plastic housing, and had just reseated it with electrical tape. Could be that you jostled something last time you (or a tech) had it open, and you've a loose connection.
Edit: Failing the above, check and clean the boards on your printheads, as ink may have splashed or leaked onto them. Generally, when there's a short on the heads, the printer will keep itself shut off as a precaution.
