Delilah Fielding is a Department of Defense employee and the current girlfriend of NCIS Special Agent Timothy McGee.
While attending a black-gala tie evening in her honor, a hijacked aerial drone (controlled by Benham Parsa) bombed the building the gala was at, killing four people and injuring numerous more, including Delilah.
NCIS Special Agent Anthony DiNozzo later revealed that due to Delilah having been hit with shrapnel from the blast, she had to undergo emergency surgery.
Although the first was a success, she later required a second surgery, which left her paralyzed, presumably on a permanent basis due to shrapnel hitting her spinal cord.
This deeply affected McGee, causing him to have a near-breakdown when he discovered this, thus forcing his reinstatement to be revoked.