People missing an entire day of gears/exp/TM/rank farming, daily login rewards and quests, arena points, and last day of the banner. How can they possibly fix that? I don't think any compensation Gumi is likely to offer will fix it.
Worst yet, if this is that serious we all might have LOST data/progress. Worst possible scenario. At this point it wouldn't surprise me if they lost a large amount of data and did not take a recent backup or prepare an adequate rollback process.
Give me a ticket that guarantees 1 of each character for this lost banner. I might have been able to pull Gilgamesh today!