I don't know what to do. Yesterday, i receive a new monitor from Dell (UltraSharp U2410) I plug the monitor to my iMac (aluminium mid 2007). The iMac detect the monitor but the monitor send me a message that is telling he don't receive any signal the computer and after few second is going in sleep mode. It's the first time a use an external monitor and i really don't know what to do. Somebody have any idea ?