If the monitor shows Power Saver Mode, or Sleep Mode, or No Signal; it is because it is Not receiving a Video Signal from the computer.
The monitor cable is bad, or the computer is not working.http://support.dell.com/support/edocs/monitors/p793/En/specs.htmhttp://support.dell.com/support/edocs/monitors/p793/En/index.htmhttp://support.dell.com/support/edocs/monitors/p793/En/trouble.htm
The main problems with desktop computer failure is;
1) Computer is dirty inside
2) Power Supply failure
3) Ram Memory module/s ('Stick') are loose. Power unplugged from computer; FOLLOW Anti-Static Precautions; the Ram Memory module/s are removed, THEN reinstalled. Visual inspection WILL NOT DO.
(Also the gold plated contact pins should be cleaned with a pencil eraser. BOTH sides, ALL contact pins. Use air to remove eraser dust)
Just because you may see LED lights light, and Fans spin; means NOTHING.
Does NOT indicate the Power Supply is good.
A) If ALL of the LED's, (Light Emitting Diodes), were on at once, they would use less than 1 Watt of power.
B) EACH fan uses 2 to 3 Watts of power.
C) A typical Processor can use from 51 to 125 Watts of power. Just depends on what Processor it is. (Older Intel Pentium III's, and AMD Athlon's use less than 51 Watts of power)
This is why a Power Supply with a weak voltage power rail, can light LED's, and spin fans; but NOT have enough power to turn the Processor on.
[There are 3 main voltage power rails.
The Switched-Mode Power Supply (SMPS) in your computer is a Converter.
Converts high AC voltage, (100 to 240V AC) to 3 main low DC voltages;
3.3 Volts, 5 Volts, and 12 Volts. (DC)
In comparison two D cell flashlight batteries produce 3 Volts DC,http://en.wikipedia.org/wiki/Computer_power_supply#Wiring_diagramshttp://en.wikipedia.org/wiki/Computer_power_supplyhttp://en.wikipedia.org/wiki/Switched-mode_power_supply
Suggestion is to test the 3 main voltages coming out of the Power Supply, with a multimeter set to DC Voltage.
You can buy an inexpensive multimeter that is plenty good enough for this job; for around $5 to $12.
Borrow a KNOWN to be good, Compatible power supply from a working computer; and use it temporarily for a test unit ]
If you are getting a video signal, and it is scrambled/distorted, post back in a Comment.
For additional questions please post in a Comment.