I'm having the same problem. Tracker is fully charged, but holding down the power button (for any length of time) does not cause the tracker to turn on. The USB cable trick (as described by the other posters) does work for me, although it's not a practical solution for my work.
I noticed another strange behavior, which led to a solution for me. SteamVR was open, and my tracker was powered on and being tracked by SteamVR. Then I closed SteamVR, and this automatically powered off my tracker. Then I opened SteamVR again, and I saw that its tracker icon was fully blue (indicating that my tracker was being tracked); yet my tracker was powered off. After less than one second, my tracker's light flashed red once, and then the tracker icon in SteamVR went dark. After this, my tracker would not power on, except by using the USB cable trick.
This makes me believe that SteamVR is sending some erroneous commands to my tracker, either when SteamVR closes or when it opens.
My solution: in SteamVR Settings --> Startup / Shutdown --> set "Turn off controllers after" to NEVER, and set "Turn off controllers when exiting SteamVR" to OFF. Yes, that means you will need to manually turn off your tracker every time, but it's better than having to trick it into starting.
