Horrible-devices, "God knows" perhaps, they certainly ain't designed for user-re-tensioning. L
I'm assuming your warranty period has expired. If not, go get it replaced. I've had several vac's on which the power-cord tension has gone. It seems to be quite the deisgn fault as I don't think I mistreat the product to begin with. Also I find there isn't enough power cord as well so I use the opportunity to extend the cord and simply wrap it round the machine, or in one case, I fixed brackets onto the machine so that I could wrap the cord on it. The answer very much depends on how much trouble you are prepared to go.
Unless it has been designed to be adjusted, it would be best not to try and open it up, as most spring retracting cables contain several feet of spring, which tends to fly out as soon as the lid is opened, and is very difficult to get back in.More practically, you could shorten the power cable enough for it to get wound all the way in.