According to the Samsung Manual S24D590L has an internal Off function which and I quote
*For products for the market in Europe, the Off Timer is set to automatically activate 4 hours after the product powers on. This is done in accordance with power supply regulations. If you do not want the timer to activate, go to MENU > SETUP&RESET and set Off Timer to Off.*
You haven't said how long it was on or how long it was left but it looks as if the monitor is currently powering off and will not respond until turned on again. Check the settings and turn off the OFF timer.