Are the cord tensioners in place with the cord intact and the tensioner fastened to the wall? If not, the cord will have to be fastened to the tensioner and the tensioner attached to the wall. If the tensioners are in place the cord is probably too loose. Apply more tension to the cord by adjusting the cord in the tensioner.
The cords are probably too tight. Loosen the cords in the tensioners until the shade just barely stays in the up position. A sign of too tight cords will also be a bowed rail.
Check the tension on the cords. Both cords must have the same tension for the shade to operate properly. Check to make certain that the cord tensioners are located straight below the end of the bottom rail. Check to make certain that the top rail is located straight above the bottom rail. These items not being aligned will cause the shade to be crooked and or the pleats to run at an angle. If the above checks out correctly, you can carefully pry the middle rail end caps out. While the shade is still mounted on the wall, operate the shade up and down SLOWLY and CAREFULLY. If the cords are hung up somewhere within the rail, this will realign them. Replace the end caps.
This is normally caused by the shade being stored for a long period of time in the down or extended position. This can also be caused by the shade being wet causing pleat retention to deteriorate. In either case force the shade to the compressed position carefully guiding the pleats so they stack properly. Keep in this position for as long as you can. This may help restore the pleats. Shades with blackout lining are more prone to this issue as they are thicker and resist pleating more than shades without blackout liner.