One of the reasons the unit can fail to power, is if the pins on the SD card oxidse.  Over time, the fine dust that gathers inside the unit can work its way onto the SD card and holder pins, which makes an intermittent connection causing the unit to fail to boot.

To fix this issue, remove the rear cover of the unit, take out the SD Card and clean its and the holders pins with isopropyl alcohol or a similar cleaner that dries without leaving a residue.  Let it dry and put the card back in.  Re-assemble the unit and power on.

If the unit still fails to boot, I would advise going to the support page on the TC Helicon website and putting in a ticket to the Support Team.

While doing the above, make sure the power has been switched off at the mains.  We also recommend that this is carried out by a TC Helicon authorised service centre.