Wireless and internet access are two different, distinct things. Wireless networks are basically the communication between any local computers and a modem. Internet is the communication of the modem with the outside world.
Your wireless network can function perfectly without a modem, and you would never have internet access. The diagnosis will depend on whether or not you can see/access other computers on the network (or the modem configuration), or not.
If you can not, then it is a problem with the wireless (or local) network. If you can, there's a good chance that the modem is not configured properly or there is a loose connection somewhere. Basically, try to eliminate as many possible sources of the problem as you can by working from one computer out.
Start with another computer on the network. If you don't have one, then try to access the modem setup page through your browser. If you can't, then try to open up the router configuration page. They're usually some variation of 192.168.x.x
If you can't open that up, then try to ping it. If you can ping it, then try to ping another address (i.e.
www.google.com). If you don't see the domain name translated into an ip address, then there's a problem connecting to the DNS server, so you're not getting through to the internet.
Check your modem's status. Unplug the whole setup (modem and router), and wait for about 30 seconds. Then plug in (in this order) the router, wait about 15 seconds, then plug in your modem. Sometimes modems will bind to a particular MAC address (note that this MAC does not mean the same thing as Mac) and needs to be reset.
Other than that, call the ISP. They're usually pretty good at figuring things out. Good luck.