As was proven when Trump instituted his executive order, people with valid visas and even green cards were denied entry into the US[1]. That's since been reversed in the courts... but that doesn't mean that the current, or even a future, administration won't try again in the future. (I doubt Trump will, after getting blocked in the courts... but you never know.)
[1] http://www.pbs.org/newshour/rundown/trump-muslim-ban-green-c...