Face-down creatures have a converted mana cost of 0. If one is exiled by Silkwrap, it will return to the battlefield face up.

Silkwrap’s ability causes a zone change with a duration, a style of ability that’s somewhat reminiscent of older cards like Oblivion Ring. However, unlike Oblivion Ring, cards like Silkwrap have a single ability that creates two one-shot effects: one that exiles the creature when the ability resolves, and another that returns the exiled card to the battlefield immediately after Silkwrap leaves the battlefield.

If Silkwrap leaves the battlefield before its triggered ability resolves, the target creature won’t be exiled.

Auras attached to the exiled creature will be put into their owners’ graveyards. Equipment attached to the exiled creature will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist.

If a creature token is exiled, it ceases to exist. It won’t be returned to the battlefield.

The exiled card returns to the battlefield immediately after Silkwrap leaves the battlefield. Nothing happens between the two events, including state-based actions.

In a multiplayer game, if Silkwrap’s owner leaves the game, the exiled card will return to the battlefield. Because the one-shot effect that returns the card isn’t an ability that goes on the stack, it won’t cease to exist along with the leaving player’s spells and abilities on the stack.