WHAT HAPPENED TO JENNIFER LIEN/KES?
The Canon Answer:
"The Gift" (VOY): Affected by the psionic abilities of Species 8472 in the "Scorpion" two-parter, Kes' latent Ocampan mental abilities manifested themselves, including the ability to "resonate" matter into another reality. When her transformation into a super-powered being threatened to destroy Voyager, she left the ship and "resonated" into another plane of reality, but not before giving Voyager a generous, 9,500 light-year push.
The Company Line:
According to interviews with Voyager bigwigs Brannon Braga and Jeri Taylor, Kes was written out because they just "ran out of ideas" for her character. The parting was amicable.
The Lowdown:
Jennifer Lien was fired, but, I must emphasize, not because of drugs (as rumored) or problems with the producers--and this comes from a source who was "in the know." She was canned because they planned to bring on a new character (Seven of Nine), and having ten principal characters would have been a bit too much for them to handle. Seven's in, Kes's out--interstellar corporate downsizing. My mole on Voyager recalls that when they were filming "Scorpion II" and "The Gift," Lien was pretty much wandering in a depressed funk. Other castmembers weren't too thrilled with her departure, either.