....I do believe she committed suicide, but can't say I'm 100% sure or as to how she did it.
Someone...anyone..
I remember Rick watching a video message from her and he wondered what went wrong with her...not the same person he once knew or something. Also, he was dreaming about her and in the dream they both died in a fiery motorcycle crash. Maybe that's how she killed herself or died from the accident.
Moonlighting. I don't remember ever detail, but I do remember Maddie was pregnant by David, didn't tell him, stayed at her parents, went back to him (or he found her) and she lost the baby while at a party or something. I remember she was on the dance floor when she got sick.

After that, I don't know what happened.
Catch you later.