You can just tie a new loop. If you are not comfortable, don't. Otherwise a repair shop can sew a new loop or you can find North repair kits.
When tying a knot you must also trim the back lines to make up for the lost line length.
Double figure 8 used by climbers (one figure eight on the line, loop end back and retrace figure eight). Might use too much line but is strong and doesn't weaken the line too much.
You can secure the end of the knot with an extra overhand knot to reduce chance of slipping.http://www.realknots.com/knots/sloops.htmWarning:
It is not clear to me if the broken loop is the one that holds the 5th line red ball (I don't think it is but just in case). If it is at the red ball, it is very important that the ball cannot slip through the loop. For one thing, if it slips through, you loose the leading edge support. More importantly it can result in powering up the kite when releasing to 5'th line if the red ball cannot slip back through the loop.