When a monitor displays No Signal, it is an indication that there is no video signal going to the monitor.
The monitor is working fine.
1) Could be due to a bad monitor cable. They do go bad.
Test would be to take that monitor cable, (If it disconnects), and use it on a known to be working computer.
2) Could be the graphics of the computer.
If the computer has Integrated Graphics, the test would be to install a graphics card, and plug the monitor into it.
[Integrated Graphics is also termed as OnBoard Graphics.
On the motherBoard.
The GPU is the graphics 'engine' of a computer.
Graphics Processing Unit.
It can be soldered directly to the motherboard, (Integrated Graphics), or soldered to a removable graphics card.http://en.wikipedia.org/wiki/GPU
When you install a graphics card, (graphics expansion card), the Integrated Graphics is bypassed.
The monitor plugs into the graphics card, not the motherboard.
Could be wrong, but I'll bet it's the Power Supply.
If this problem directly relates to a Compaq Presario SR1550NX, the computer is 5 years old. (According to my sources)
If the computer isn't cleaned inside, and the Power Supply isn't cleaned inside, on a regular basis as needed, the Power Supply is one hardware component that is sure to fail.
It's the top problem relating to computer failure.
The cooling components for a power supply is it's fan, and Heatsink's that are used inside.
The basic physical makeup of a Heatsink is a flat plate of metal, with tall, thin fins protruding from it.
The flat plate absorbs heat from whatever it is placed against, and the tall, thin fins absorb the heat from the plate.
The fins then radiate the heat away.
If there is a fan used along with a Heatsink, (Such as in the Power Supply, or on a Processor), the fan moves air in-between the fins, and around the fins, helping to carry the heat away.
If dirt, dust, hair, etc. is clogged on the fan blades, center hub, and surrounding cage, the cooling capacity drops tremendously.
The fan cannot move the needed amount of air.
To include on the Heatsink/s inside the Power Supply.
The gunk acts like an insulating blanket, causing the Heatsink to retain the heat, not conduct it away.
Heat = Wasted Energy
The Power Supply tries to keep up with the call for power, and eventually cannot, due to hardware components failing inside.
The Power Supply may seem like it is operating okay.
Lights light, fans spin, and it may sound as though the Harddrive inside is operating.
The Power Supply could have a weak Voltage power rail.
Power Supply's are rated in Wattage.
1) ALL the lights use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts.
3) A typical Processor can use anywhere from 51 to 125 Watts.
A bad Power Supply with a weak Voltage power rail, will have enough power to light lights, and maybe spin fans, but not enough power to turn the Processor on.
Harddrive sounds like it is working, because the Platters inside are being spun by the Spindle motor.http://computer.howstuffworks.com/hard-disk.htm
There is no Processor operating (The 'Brain') to find the boot sector on the Harddrive.
Platters are spinning but the Arms aren't moving the Read/Write heads across the Platters.
In short the computer is not operating.
No video signal to the monitor, because there is no computer operating to send a video signal.
Bad Power Supply.