Dell U2413 User Manual page 54

This means that the monitor cannot synchronize with the signal that it is receiving from the computer. See Monitor Specifications for the Horizontal and Vertical frequency
ranges addressable by this monitor. Recommended mode is 1920 x 1200 (for the U2413)/2560 x 1440 (for the U2713H).
You will see the following message before the DDC/CI function is disabled:
When the monitor enters the Power Save mode, the following message appears:
Activate the computer and wake up the monitor to gain access to the OSD.
If you press any button other than the power button, one of the following messages will appear depending on the selected input:
DVI-D/DP/Mini DP/HDMI input