why wont my laptop stay connected with the internet?

Hi

I am currently using a 15 " Mac Book Pro from 2011. I am using a Telecom Router (german device) and since yesterday my laptop wont stay permanently connected with the internet. I never had a problem like this before and the internet works fine on other devices (mac book pro 13" 2010, iphone 3gs and ipad 2, also an old windows pc). Can anyone help me find a quick and easy solution for this problem? I dont know much about computers in general, so please explain it, like you would talk to a 10 year old! 😉

i am using a different apple id than i usually do, so dont get confused with the products!

thanks a lot and greetings from germany!