I missed some of the first episode,n but I've Googled it and he's written out that he's retired from SVU, but Benson didn't get a chance to say goodbye to him.
chris meloni (stabler) fell out with the producers over his fee. Long story short,they wrote him out in a back story that he was being investigated (again) over a shooting of a real criminal that he shot,and that ended up with him handing in his badge.(all that was unseen,btw, just reported to olivia)