Often jobs are deleted because they're scams/fraud/violate the ToS somehow. The client could have changed her mind and deleted the job herself, but usually when you go back to a job posting and it's been deleted that's not the case.
@Vanessa L wrote:
I'd like to know why a specific job I interviewed for was deleted. Can someone on Upwork help?
You might consider yourself lucky. Usually when a job is deleted, it's probably some sort of scam. Before looking and applying for jobs, you should read through some of the posts here, to acquaint yourself with the scams that go around here. So in the future, you don't get caught in one.