Issues & Solutions
Issue #1: Wireless not turned on
Sometimes, especially on laptops, it's easy to bump or accidentally turn off the wireless button. With the wireless turned off, unless you are connected via a cable, you will not get internet access. It is also possible that you may be in area that does not have wireless connections.
Solution - Check to make sure that the wireless feature is turned on. If using a laptop, make sure that the wireless button or switch is in the on position. Usually, when this feature is on, a light will appear on or near the button. If using a desktop, make sure your wireless card is enabled. Here's an article that describes this. Also, make sure that the area you are in has an active internet connection.
Issue #2: Computer not connecting
Perhaps one or both computers aren't connecting period, despite having the wireless connection turned on. So why won't the computer not connect? This problem has several issues and solutions. Perhaps there is an issue with the hardware itself or maybe there's an outage happening with your ISP.
Solution - Try calling your ISP (internet service provider) to see if they are having an outage. Certain areas may experience problems due to weather or wiring or even accidents. Sometimes, if there's an outage with the electric company in the area, there will be issues with the internet lines, regardless of if your power is still on. See the next issue and solution for hardware.
Issue #3: Hardware
If issues 1 and 2 don't seem to be the problem, perhaps it is the hardware, namely the cable modem or wireless router. If the router is having issues, you won't be able to connect wirelessly, but you may be able to connect with an Ethernet cord. However, if the modem is having issues, both wired and wireless connections will be inaccessible.
Solution #1 - Try unplugging the power from both devices for a few seconds. Next, plug in the cable modem first and allow all lights to come on. Next, plug in the router and again, wait for the lights to come on. If you notice that the cable light on the modem hasn't come on or flashes for a long time before becoming solid, it may mean two things. Issue #2 may be in affect, so try giving your ISP a call. They can also reset the modem from their offices to ensure a connection. The second possibility is perhaps the cable modem is on its last legs. A simple replacement may be needed.
Solution #2 - As mentioned, the hardware may need to be updated or maybe replaced. Try searching the manufacturer's website to see if new drivers or firmware is listed for your model. Oddly enough, routers and modems don't really last for very long, though it may depend. If you've just bought a new router or modem, it may be faulty. Make sure to keep the receipt and packaging so you may return it to get a replacement or your money back.