I thought it was the power-cable's problem - plugging loosely into the monitor. So I've changed the power cable, which didn't solve the problem If I hold the cable's plus up, to keep the connection to monitor steady and tight, the light on the monitor goes green, the sign "No power supply" shows on the dark screen, then the green light on the monitor changes to orange light.

Computers & Internet - Planar - PX2211MW Monitor