Jim's suggestion of changing channels is the best bet for a quick solution.
But if you find it happening again after trying different channels, here are a few other ideas:
--Reboot the router (always an easy step to try)
--Click on the wifi symbol on the top tool bar
Select "Open Network Prefrences..."
Click on the "Assist Me..." button at the bottom.
Click on the "Diagnostics..." button on the pop up
Select Wifi and Press Continue
Select the Network Connect and Press Continue
Wait a bit and see if it connects on that attempt
--I found this about people reverting to the kext file from Snow Leopard which is apparently more friendly than the one in Lion:
"Some Mac users who upgraded to Lion (10.7) discovered that the wireless connection is unstable. It'll drop or disconnect you randomly. Or what's worse, the Airport icon will show you have full bar reception, but you cannot load any websites.
The issue is due to a bad kernel extension file (kext). Think of kext as "drivers" in the Windows world. Lion has a new set of kext files, in which the wireless kext files are not working normally. The fix is to use the old Snow Leopard (10.6) kext files because your wireless must have been working before.
Instructions:
1. Download the kext file here:
http://www.sendspace.com/file/jy1b4k2. Unzip the file
3. Drag IO80211Family.kext and drop it to Kext Utility. This will launch Kext Utility and update your kext file.
4. Turn off your Mac when the program is done. Wait 1 minute and turn it back on.
Note:
DO NOT just double click Kext Utility. You must drag and drop the .kext file. This tells Kext Utility to use the .kext."
---Another idea:
"Go into your routers setup and locate the advanced settings. Find the option for UPNP and disable it. "
Each of these suggestions helped people but none was celebrated as a universal fix.
Oh, then there is always... "Buy a new router." (cheaper than a new Mac)