How come a retina is completely detached?

Usually a tear. Retinal detachments can be fixed with surgery with significant return in vision. See a retinologist ASAP. .
Neglect. The retina can become detached from its normal position by a variety of causes. Once started, if untreated, it will usually completely detach leading to profound visual loss. this will happen if the symptoms such as light flashes, floaters, greying of vision and venous patterns in the vision, are ignored.
Retinal detachments. The retina is permanently attached to the optic nerve and the ora serrata( the area in front of the retina). A total detachment means the retina between these attachments is separated from the lining under the retina(rpe). This can be from a tear that lets fluid under the retina (rhegmatogenous) or from fluid leaking under the retina (serous). Exceptions - giant retinal tears and on avulsion.