I have a Pioneer PDP-R05G media receiver and PDP 435-PG plasma display. After power up the tv turns off after about 2 seconds, and the media receiver green power light flashes 8 times then pauses and cycles again while the Plasma display power light returns red (as if on standby). the unit has not been used for approximately 3 months but was working fine previously.

This is a classic "Hic-upping" switchmode power supply problem. The protection circuitry inside the SMPSU, has sensed a severe potential problem, and is attempting the shutting of the set, down to avoid a fire, explosion, or both.
This set had developed a problem, that is so severe that the set must be shut down, (This set should be removed from the Power),
What is happening is the internal "Fault" that is causing the Protection Circuitry , to detect the problem and attempt to shut down the set, keeps restarting the set, because as soon as it shuts down, the "Faulty" component(s) cool down?, and the "Fault" ostensibly "Clears" so the PSU starts up again, only for the component(s) to heat up, whatever, again and fail, so it shuts down, and if left unattended, this would continue on until the part eventually failed. As related this is called a "Hic-upping Power Supply". Any fault that involves the power supply is usually quite serious, and needs a professional, usually, unless one is familiar with servicing current day, televisions.
And in my professional recommendation, you should get a "Quote" for repair, from, an, Approved Service Center. Now, this point is very important for a good result, Always contact, their "Local" or a, "Head Office" Philips in your case. Ask them, for a referral to an authorised servicing center, where you ask for a "Quote". Now, when you get the information you want, then you can decide as to repair of replacement.
The reason we want an approved agent, is that way, all the engineers are trained on their own brand sets, they have seen almost all faults, and will get the job done quicker, and for less cost, and usually have parts on hand, also sometimes, there has been a service bulletin in issued and the repair may be subsided or even free. There are many, many, many reasons that display this symptom, one must work through the troubleshooting steps to isolate and identify, the Cause and repair that and any damage done.
http://www.pioneer.owner-manuals.com/PDP4360HD-service-manual-PIONEER.html
service manual link above,
as an aside here, sometimes things in storage can quickly degrade especially big low value capacitors, these increase in internal resistance, and all sorts of problems ensue, this is why, say, military things are cycled at least weekly when in "Moth balls" to prevent exactly this.