How do I connect to internet?an

Recently had Apple upgrage my Mac book Pro to El Capitan.

When I turn on my computer, the WiFi login box does not appear any more so I cannot enter the password. In addition, under network preferences,it says "WiFi has the self-assigned IP address and will not be able to connect to internet" I am a small entrepreneur and have lost 3 days of work trying to figure this out. All forums give different advice and many are too technical for me.

PLEASE don't send me to another Forum. I am happy to try solutions again from you. Thank you in advance.Deborah