My Laptop won't connect to wifi but ethernet works. How do I factory reset w/out reset disk?

Okay so I got this laptop from a friend, and he told me he was cleaning it and deleted a few things and messed some things up on it but he said it was only things like viewing pictures. So anyway when I asked him for the factory reset disk he said he didn't have it, he had lost it. When I turn on the laptop the icon at the bottom left of the screen tells me there is no Internet connection available. The Ethernet cord connects the internet fine, and the internet connection becomes available, but I'm still unable to use the Wifi. Also the ethernet cord is loose. It doesn't come completely out of the socket but it moves back just enough so that the prongs no longer match up. How do I fix that?

This is an issue with my laptop, because all the other devices in my apartment can connect to the Wifi, and the modem is showing all the appropriate lights and such. I've tried turning it off and on again and resetting the modem, and it made no difference.

My laptop is a Dell Inspiron 5520, and the system I use is Windows 7.

My modem is TARRIS
 
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