Asking someone out and having them say no doesn't necessarily mean no forever and always, it just might mean 'no, I'm busy/in a situation where I can't date at the moment'. Feel it out. Is there a relationship status in the way? Is she newly dumped? Etc.
Generally if someone says no multiple times, no means no. But one no might just mean "I already really do have plans" not necessarily "I'm not interested."