Apple menu > System Preferences > Network > click Assist Me... button. The red "Failed" markers will tell you a lot. If something is failed, that's the thing that needs to be fixed. ¿Claro? A network connection works in a series, a chain of:

computer > router > modem > Internet.

So if one bit is bad, there's the break in your chain. But here's a little more translation of the signs: If your computer says it doesn't have an address, then it's your router. If your router says it doesn't have an address, then it's your modem. If your modem says it doesn't have an address, then it's your Internet. You are responsible for the computer and router, and your Internet service provider is responsible for the modem and Internet (unless your ISP is bandito originel, pinche AT&T, who makes you buy a modem).