If you don’t play the card, it will remain exiled.

Playing the card exiled with Hedron Detonator follows the normal rules for playing that card. You must pay its costs, and you must follow all applicable timing rules. For example, if the card is a creature card, you can cast that card by paying its mana cost only during your main phase while the stack is empty.

Unless an effect allows you to play additional lands that turn, you can play a land card exiled with Hedron Detonator only if you haven’t played a land yet that turn.

You may play the card exiled with Hedron Detonator even if Hedron Detonator leaves the battlefield or you lose control of it.