It sounds like you're just referring to the LUN size here vs. the other kind of "quotas" available in ONTAP? Anyway, ONTAP takes your LUN offline when it fills up to protect data integrity. Initiators might do unpredictable things to the data if no more space is available and the "safest" course of action for ONTAP to take is to take the disk offline. This is a couple of years old, but the concepts still apply:
It sounds like your LUN has gone offline because the volume containing it has become full. This usually happens because of snapshots being taken of the LUN. The default behaviour is to retain all data and snapshots, and take the LUN offline, but you have a couple of options to prevent this.