Samsung LS21DPWAB-XAA - Manual (Page 67)

then contact the Service Center or your dealer.
z
Judging the monitor's working condition
If there is no image on the screen or a "Not Optimum Mode","Recommended Mode 1680 X
1050 60 Hz" message comes up, disconnect the cable from the computer while the monitor is
still powered on.
{
If there is a message coming up on the screen or if the screen goes white, this means the
monitor is in working condition.
{
In this case, check the computer for trouble.