hold the power button down and plug in the set. if it powers on, short the power button and call it good.
if it won't turn on when plugged in, try placing a 100 or 200uF (16 volts or higher) capacitor across the power button contacts. you will have to measure to see which contact is supplying the 5 som odd volts used to sense the button press and put your + lead of the capacitor on that lead.
that capacitor will act as a short on the button but "look" as open once the capacitor charges up from the 5 volts, essentially acting like a button press.
some TV's will just power on again if powered off while they are active. you may not need to do anything.