Mara | Leia
SHIELD doesn't exist any longer. Earlier this summer it was revealed that a group called HYDRA had infiltrated it. Steve Rogers, he's known as Captain America as well, was one of the people who discovered it. He was on a wanted list for a while, but that was because HYDRA was heading SHIELD. The organisation fell apart and those of us who were loyal to SHIELD not HYDRA had a bit of a rough go for a while. HYDRA mostly disappeared although probably not entirely... It's been a bit confusing for the past few months.
That the shortest answer I can give you. I didn't really leave SHIELD, but I'm not employed by them any longer.