Writing permissions denied

hi for all,

i am using my cloud (4TB) for 2 years.
yesterday i met for a first time a strange problem-
i downloaded another 50 GB of data (to 95% of total capacity) and suddenly didnt have writing permission!
i cant delete/edit/save to the disk. i tried to reboot to disk, checked the user permissions and it seems ok.
i am using win10.

can anyone help me??