Try unplugging and plugging back in the router and your computer connection - for at least 30 seconds. This should allow the internals and the dns cache to reset.
Then restart the computer and see if it'll reconnect.
If not, check your anti-virus and your windows (assuming you have windows) configuration. You may have on a double firewall. Sometimes these will conflict (ok it's really quite often) after installing new software and/or updates. (Most updates contain the firewall "on" option as the default. ) If this is the case, it can cause you to be unable to connect to the internet and/or have weird problems with modems/etc.
If you are unsure how to check, you may want to simply disable the anti-virus you have and try going to a safe page (think a home page like
google or yahoo). DON'T SEARCH - don't open any attachments, Don't download anything (you know the stick - do not pass go, do not collect $200...). Just see if you can get there. If you can, then it's most likely a problem between the two - your operating system firewall and your anti-virus firewall. From there, Pick one and re-configure it.
Hope this helps.