This only happened once and one of our costumer loose 11 Tb of ISCSI lun. Worse part is, lun parts are not exist under .@iscsi foldern on the device. I contact with Qnap Taiwan and request help, but they said there is nothing we can do, because files are not exist anymore.
You maybe not belive that but, after 3 days of research, I found whole ISCSI files under /share/MD0_DATA/lost&found, and folder name was “117833729″
I change folder name with .@iscsi.img and move it under share/MD0_DATA, and whole ISCSI comes back.
If you loose your ISCSI, just check device folders carefully if its sitll exist or not.