I've had this problem happen 4 times overthe course of a year and 5 months with my Wii U since buying it. You know how the Power Light is normally Red, but will turn Orange/Yellow when the Wii U is doing something in Standby Mode? For me, on four different occasions, the Power Light has turned Orange/Yellow and stayed Orange/Yellow. When I tried to turn on the Wii U, the Power Light will turn Blue, but the Wii U itself will not start. The fan nor the Disc Drive will make a sound. The Wii U will sound as active as it was when it was turned Off, with the exception being that now the Orange/Yellow Light is Blue. To fix the problem, I have to physically unplug the Wii U from the back. Holding the Power Button down for 10 seconds does nothing. After replugging the Wii U, the Power Light will be Red and the console will resume functioning as normal.
The first three times, I was unsure why this was happening. However, the 4th time, I think I was able to narrow it down. The Wii U has done this when the console has a pending update to install. The 3rd time this happened back in November, I had Mario Kart 8 data that had been downloaded, but not installed and last night, there was the 1.1.5 Smash Bros update.
I'm not sure if this is the exact cause, but I'm positive it's related to Standby Mode Downloads.
My Wii U console is updated with the latest software, so I know it isn't due to a console update.