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Apparently this is a huge issue that's been going on since launch, for some reason tonight (with all the server instability - talk about a bad night in ESO - chat lag, cyrodiil unresponsive ... ugh)
It seems to hit folks that have upgraded their bank slots from 80 and beyond.
Basically you lose EVERYTHING stored in bank - and then your bank is reset to 60 slots!
There is a huge thread on official forums - ZOS apparently has no way of fully restoring your bank items - so you could lose all of those rare mats forever.
This is not an isolated issue folks - my advice - DO NOT UPGRADE BANK SLOTS til this is sorted out.
EDIT - One good suggestion is to take screenshots of your bank inventory - ZOS could use them to grant back lost items/cash and lost bank slots.