There is a tool button that is called toggle segment .  It is next to the scissors tool button .   If you use this tool on the part of the line that was cut, then it should toggle back on.   

In fact, what the scissors tool does, is to insert two points on the line, and then toggle off the segment between those two points.  This is why you can reverse the process by toggling on the segment again, and you can use the Remove point  tool to remove the two points again.  

You can also use the Insert point  tool together with the “toggle segment” tool to create smaller or longer gaps in the line.