Depression can have a huge impact on sexual function. Many people dealing with depression lose interest in sex either due to the symptoms of their depression (that "sudden loss of interest" thing we always hear about in medication commercials) or due to medication side effects.
The thing to understand is that the issue here isn't sex. The issue is depression. Is your partner being treated? Can they address their loss of sexual interest with their doctor? Effectively treating the depression may be the key to helping your partner feel sexy again.
In the meantime, you can help by listening to your partner. How does she feel? What does she need? Encourage her to make conscious decisions regarding sex. Why is she not interested? Is her mind not there for sex at all or is she interested, but sex sounds demanding and onerous (because when you're dealing with depression most things do)? If it's the latter, the two of you could discuss ways to make sex feel less demanding for your partner. Depression can impact our sex lives, but it needn't end them.