There are soooo many things this could be. I will give you every idea I can. Respond to this post whether these steps work or not. If you do, I will continue working with you on this until you can connect.
1.) Can any other computers connect? If you have other devices which can NOT get online, the issue is with your modem/router. Unplug the power cable(s) to your modem and router (if you don't have a router, disregard anything I explain regarding a router) AND SHUT DOWN your PC. Wait about 1 minute and plug the power cable into JUST YOUR MODEM. Wait 3 minutes for it to get a connection. Look at the lights on your modem to ensure they look normal (the "ethernet" will be OFF. That's OK.). After 3 minutes has passed, plug the power cable into your router (IF AND ONLY IF you do NOT have a router, turn ON your PC). Wait another 3 minutes and turn ON your computer (If you have no router, you just did this and it may be disregarded.) ---- In case the whole PC/router makes little sense: If you have no router and just a modem and a PC, then 3 minutes after you plug the power cable back into your modem, turn ON your PC. If you have a router, then follow the instructions exactly). ----
2.) Check for a switch on the left edge or front edge of your laptop which says "Wireless" or "Wifi." Be sure it is in the ON position (Not every laptop has this. Some have a wireless button above the keyboard and others have nothing at all.). It is very common for a user to accidentally bump this switch to the OFF position without realizing it.
3.) Perform a System Restore to take your PC's settings back to when it was working. Click Start/Programs (or All Programs)/Accessories/System Tools/System Restore. Go through the System Restore wizard to select a date BEFORE this issue happened. System Restore should NOT be interrupted. It can take up to 20 minutes in some cases. When it begins, set a timer for 20 minutes, walk away, and come back after that time has elapsed.
3a.) Windows XP: http://support.microsoft.com/kb/306084
3b.) Windows Vista/Win7: http://windows.microsoft.com/en-US/windows7/products/features/system-restore
4.) Clear your network settings from your PC and then reconnect to your network (if you do not know yourSSID (network name) and wireless network key (wireless password), then do not do this step. Instead, respond to this answer with the brand of router you have as well as the operating system you're using). This is where it would be nice to know which operating system you're using. I'll do my best with this while not knowing your OS.
4a.) Windows XP
: Click Start/Control Panel/Network Connections. Then RIGHT-click your connection labeled "Wireless Network Connection [#}" and do a normal left-click on Properties. The network properties window will open. Click on the upper tab labeled "Wireless Networks." Under the Preferred Networks section, click the network you're trying to connect to and it will highlight. Then click Remove (immediately below the list of networks). If you are not sure which network in this list you are trying to connect to, repeat this process until that white box is empty. Then click OK. Again, RIGHT-click your connection labeled "Wireless Network Connection [#}" and do a normal left-click on Properties. Click on the upper tab labeled "Wireless Networks." Then click the button "View Wireless Networks." You'll be brought to a window where you can choose a wifi network with which to connect. Click the network you want to connect to and it will highlight in blue. Click "Connect" in the lower-right corner of this window. It will most likely (not always) ask you to enter your Network Key (network password). Enter this exactly; capitalization (Big letters and little letters) matters!
4b.) Windows Vista/Win7
: Click Start/Control Panel/Network and Sharing Center. Click on Manage Wireless Connections. In the list, click the network you're trying to connect to and it will highlight. Above the list, click Remove. If you're not sure which one is the network you're trying to connect to, then repeat until all of the networks are removed and the space is empty. Once done, goto this link (CAUTION!!! I would right-click on this link and choose "Open link in new tab" OR "Open Link in New Window" to allow these instructions you're currently viewing to remain open): http://windows.microsoft.com/en-US/windows7/View-and-connect-to-available-wireless-networks
If none of this works, respond to this answer and I'll help further. I hope this helps!