Sleep mode is a power-saving method that a computer will frequently go into in order to prevent wear and tear on the components when the system hasn't been used for a while. A computer can be told to go into sleep mode by the user or it may do it automatically after a set amount of time without activity. Dell computer monitors also have a sleep mode to prevent the pixels from burning out when the computer is not being actively used. You can take a Dell out of sleep mode with a few clicks.
Press any key on the keyboard. This should cause the monitor to power back up. You may need to enter a password to completely take the monitor out of sleep mode.
Turn off the power on the monitor by hitting the power button. Wait a few seconds and then turn the monitor back on.
Unplug the monitor and then plug it back in if it is still not coming out of sleep mode, and check to make sure the computer itself is turned on and that the monitor is connected properly.