The power cut was probably the cause. It might well have flickered a few times before going down, causing the router to reconnect at a low speed.
But the IP profile should recover within 5 days, and in a case like yours where the difference in speed is so large, it should happen more quickly than that. So I think you have to talk to your ISP and persuade them that your profile is stuck. You'll probably have to go through all their help centre scripts first, so you'll have to be patient. Perhaps if you point out clearly that this was a temporary glitch caused by a power cut, it might help to bypass some of the tedious stuff.
