Can I lose like half of my power after 3 weeks of non training
no.
Aerobic fitness goes first when you stop training.
Unless you have been paralised ;D
Try to be a bit more helpfull, what is it you are trying to do?
yes lying on your back will hinder your power as will as good dose of flu.
Your stamina probably goes down the most. But if you are still active (you do a lot of walking around and not laying on your ass) you will not get out of shape as fast. But I still wouldn’t suggest not training, unless your going on a big vacation or something. O0
no excuse 
take some runs around where ever you go and take in the scenery 
theres plenty of stuff you can do in your room with no equiptment to atleast maintain your strength too