buy a flaring tool kit and a new fitting. cut off the flare just enough to slide the fitting off, slide on new fitting and reflate. if the line is in good condition and you didn't cut too much, you should be able to rebend the line with maybe only ½" loss in the line.
if the line is at all rusty, i suggest getting a replacement line if there is a junction block close by or make a new line from a point on the old line that is in good shape. you'd still need a new fitting and flaring kit but also a union fitting and a pre-assembled ready to bend line. most decent auto parts storers should carry these things.