My Note 2 recently started apparently locking up/freezing and apparently required powering off to fix it.

Thanks to this post, I discovered that this seems to be a "known" problem with the eMMC chip which is susceptible to "Sudden Death Syndrome"

There is an app to determine if your phone has the chip that is affected, and another app to write data to every area of the chip to "fix" the issue.

My phone now appears to be back to normal.