|You say: "The modem and screen are both on " - presumably both are powered directly from a mains outlet.|
Are there any indicator lights on the desktop unit itself?
Any indicator(s) of hard drive activity?
Any sounds of life (as in fans powering/running up?
You obviously have moved the unit from one location to another. Quite possibly something may have become displaced in their sockets/connector...?
Might be useful to remove the power cord; open the case and carefully reseat all internal connectors?