Actually in the books Lynley never quit, but after **spoiler alertHelen's murder he did go on bereavement leave.

I don't know about the show, but in the books he quit.

If I remember correctly, he was suspended for beating up a suspect and dangling him over a second story railing. During his suspension he butted in on Barbara's case, which she shared with a female DCI, and had to be reminded a couple of times that it wasn't HIS case. He was reinstated at the end of that show when his superiors decided he'd learned his lesson.

I'm glad I'm not the only one who is clueless! I just went back and read the synopsis of the last episode of the series preceding this one. I remember the episode, but not the incident that got him suspended.
"Life is so f-ing good I can taste it in my spit."