Monitor in permanent sleep mode

My computer was left on for a long time and the monitor was put into sleep
mode. LED yellow, screen black. Moving the mouse or hitting a key fails to
do anything. Hooked up another monitor and everything works perfectlt. Took
the old monitor and plugged it into a regular household outlet with no
computer connection. The monitor powers on but still with black screen and
yellow LED. It appears to be in some sort of permanent sleep mode. How do I
wake it up???