Care to share which OS you are using and which software you updated?
Try the basics first:
Change your router channel number. Most times this works & is all you have to do.
Disconnect & reconnect your modem. unplug it for about 10 seconds. Plug it back in. Do the same for Apple’s routers. Wait for everything to reboot.
Click the Assist Me button.
In the next window that pops up, click the Diagnostic button & do the necessary.
Research Knowledge Base for network problems that pertain to the OS that is currently installed on your computer. See these basic networking KB Articles: http://support.apple.com/kb/HT1401 AirPort troubleshooting guide
http://support.apple.com/kb/HT2712 Using network locations in Mac OS X
http://support.apple.com/kb/HT1714 Solutions for connecting to the Internet, setting up a small network, and troubleshooting
Also, run the Airport Utility app which is located inside the Utilities folder.
If using Apple's Airport, please re-post over in one of the AirPort Forums.
Wifi problems seem to crop up a lot on here and unfortunately there doesn't seem to be a 'one fix' solution.
However, sometimes any one of the following can help although, unfortunately, not always - in no particular order:
- Check wifi security - if it's WEP change it to WPA or WPA2 (preferably, as it's more secure)
- Change the wifi channel on the router
- Create a new Network Location: http://support.apple.com/kb/HT5289
- Reboot the router
Also take a look at this link and the links within the article for other ideas. They mainly reference Lion, but still seem to be relevant to Mountain Lion:
...and this link:
What version of OS X are you running and what Mac?
I had the same probelm and tried everything mentioned above. Only cure was to use an external wireless adapter to connect to my router. I used ethernet, but you could also use a usb wireless adapter as well . Problem stems from software update screwing up internal airport wireless adapter. If trying any of the above mentioned fixes doesn't do it for you, try my suggestion.
Sorry you got nailed by this issue. For me it is the only blemish on a great experience with Apple products.