In the first episode of the 7th season we learn that Stella moved to New Orleans to be the head of the crime lab there. They didn't make an episode which show her leaving. She just sent a gift to Lindsay and a letter describing her new life.
Melina didn't renew her contract, that's why they didn't even have her doing the voiceover when Lindsay reads the letter. But yeah, that's how they explained it, and why Peyton didn't return again like she did in season 6. They were going to have her in it again but when Melina wasn't coming back and they cast Sela, Peyton never came back :( I miss Stella