According to Rule 2.0 of the MLB rulebook, a strike is a legal pitch when so called by the umpire which
(e) touches the batter as he strikes at it.
(First link below - scroll down to definition of strike)
having found that. we need to see what happens to the ball. For that, we need to refer to Rule 6.08 (b) (approved ruling)
When the batter is touched by a pitched ball which does not entitle him to first base, the ball is dead and no runner may advance.
(second link below, again, scroll down to Rule 6.08 (d) and get the approved ruling)
Conclusion - the pitch is called a strike, the ball is dead, and runners must return to their original bases.