Upright the Devil is all about addictions, about the temptations/obsessions that lure us, capture us, and which we, ourselves, refuse to escape even though we have the key to our chains. In a positive sense, it can also mean letting go of inhibitions and puritanical restrictions - sometimes we need to live on the wild side, be excessive and indulge. So what happens if we reverse this?
1) Opposite: It's natural to think that the opposite of the Devil - entrapment - would be freedom, happiness, etc. But reversals are never so easy. At its best, this opposite might indicate a near escape from entrapment, a temptation resisted but only just barely. The querent chooses to keep to their stricter morals or gets a break from whoever/whatever is tempting them, so they do not indulge, and they escape being chained to the Devil.
2) Blocked: The energy of the Devil is life on the wild side, indulging, releasing, giving in, going overboard. Reading this card as blocked could indicate that the querent is too restricted in their life or restricting themselves. That they are too afraid of giving into temptation, too afraid of addictions, to the point where they are taking no chances.
This can include a fear of committing to others, of even falling in love. Keep in mind the connection image-wise (in the Rider-Waite at least) to the Lovers card. If the Devil card is the Lovers gone wrong, then the Devil reversed can be a fear of the relationship going wrong, to the point where the querent won't take a chance.
3) Upside-down: Turn the Devil upside-down and the chains fall off the necks off the man and woman, and the Devil himself is dethroned. The querent was in danger, being reeled in by someone dangerous. They held out, however, or changed their mind, eluding temptation and enslavement.
Curious about this card when upright? Go to our Tarot Card Meanings.