I have a Philips 42 in. Plasma Television model#42pf9956/37. When I turn it on it goes right back off. What the problem.

The TV is going into Protection Mode. When a failed part occurs inside the TV, it will turn itself OFF automatically to prevent further damage to other components. I'm sure the red light is blinking a code also.
Most likely the problem is occurring on the Backlight Inverter Board or the Main Power Supply. The Backlight Inverter Board is what brings power to the screen. The Main Power Supply is what brings power to all the circuit boards in the TV.
I would use the warranty if there is any to fix the TV. The repair shop may charge a crazy price to fix the problem.
If you have some electronic experience and you want to fix it yourself, you can sometimes find a Service Manual over the internet for less than $20. It will have red light blink codes, diagrams, schematics, and part numbers for every part on the boards. Replacement parts can be found by using the part numbers for the search on the internet. Most parts cost less than $10. Whole replacement boards can be found at a good price at Shopjimmy.com.