98% of the time most technology issues can be fixed by a simple power cycle of your equipment.  Try unplugging it from power waiting five to ten seconds, then plugging it back in.  

I have to ask people constantly at work if they have restarted their device when they ask me questions.  Like I said, the majority of the time that fixes it.  

If the power cycle doesn't work, you are going to want to try to gain access to the device that is supplying your WiFi.  I am assuming your question is about your home wifi and not wifi you are trying to get in a public location.  

If you are able to gain access to your wifi device, check to make sure all of the settings are okay in there.  You should see an area where it tells you want the radios are doing and you should also see an area where it tells you if you are connected to the Internet.  This would usually be through cable modem or DSL or whatever service you subscribe to.  If you don't see any incoming signal, try restarting that device as well.

If none of those things have obvious solutions, you might be looking at having to replace the wireless router.  They do sometimes go bad over time.