It loses power? Do you mean the brush roller does not spin or do you mean there is a loss of suction? Or does the vacuum shut off?
I interpret this to mean no suction. That implies no air flow.
https://bestsearchadvisor.com/miele-vacuum-troubleshooting-problems/
Lack of suction suggests either a blockage OR an air leak. If a canister is not properly seated, a gap offers air leakage. If there is a leak in the hose, again, there is a loss of suction. A hole in the hose requires hose replacement.
If this problem is not one of loss of suction but rather loss of power in the brushes, there are other considerations, such as a build up of long fibers in the roller.
https://howtofixit.net/miele-vacuum-troubleshooting-guide/ Alternatively, if the brush is failing to spin, the motor for it may have failed even though the suction continues to work.
If there is a loss of all power when you push it forward, perhaps there is a problem with the cord, a breakage where movement breaks the power going to the vacuum.
Here is a link to Miele's own troubleshooting page:
https://www.mieleusa.com/c/troubleshooting-159.htm