Do you have a schematic? Plasma TVs typically have very complex supplies with +190V, +60V, -180V, +12V, etc. and the start sequence on this one may involve checking voltages before activating the drivers. I would stick a scope on anything that looks like it's coming from the controller to switch on the main power to see if there's any activity. If it looks like it is trying to switch it on, and then shuts off again, check to see which voltage is not coming up during that period, then troubleshoot that section.
Also, a Dell plasma has a switched +3.3V for the main CPU - if yours is like this, it will either need that 3.3V constantly available or need it switched on by a separate +5Vsb-powered controller to get the processor up and running.