Sounds like it wasn't stretched tight the first time. If the seams aren't coming loose, it should be able to be pulled tight again. If you need to ask about whether you need a pro or not, you have obviously never kicked in a carpet. Go with a pro.
The carpet can be restretched, but as Teetorbilt says, hire a pro. It may look very simple watching a carpet layer stretch a carpet tight, but there is much of a learned knack to knowing where and in which direction to pull it for it to lay flat. There is also the posibility that a seam will have to be cut and remade to get rid of the slack.
Knowledge gained from personal carpet restretching failure.