One of the prominent problems that people have after upgrading their systems or otherwise modifying their systems is when their Wi-Fi connections will not work properly. The Wi-Fi connection may turn off after a brief period of connectivity, or it may just immediately shut down. At other times it may stay on but not receive a proper configuration from the router and be given a self-assigned IP address.
When these problems occur, generally people try various fixes like running general maintenance routines, but while doing this cannot hurt (and may have some secondary benefits), clearing system caches and resetting the PRAM and SMC on systems usually does not have an effect on the Wi-Fi configuration. A better first approach to network-based problems is to go through basic troubleshooting steps to see under what circumstances the issue persists and whether it can be overcome by using alternative boot and log-in scenarios.
In addition to troubleshooting your Mac, also consider that your router may have undergone a configuration change. Generally routers themselves are not the issue, especially if you can properly establish connections with other Wi-Fi devices. Still, it's worth checking. Some routers are set up to automatically apply firmware updates, after which connections to certain devices may be less stable. At other times a power outage or even a bug in the firmware may result in an odd configuration that will begin rejecting connections from some devices. Try resetting your router, checking its configuration, and even applying an updated firmware if available.
Another method for ruling out the router is to install another copy of OS X (preferably a different version than the one you have) to an external hard drive and boot off of it. If Wi-Fi connections work fine in that scenario then you know the problem is with your main boot operating system. Similarly, if you have Windows installed in Boot Camp or otherwise have alternative OSes available, you can restart your Mac under those operating systems to see if the problem is more with your hardware or network, or with the Mac OS.
Since usually network problems involve the OS configuration, one potential cause for Wi-Fi related problems is corruption in the configuration files used for managing the network interfaces. If there is corruption or misconfiguration in these files, then updated system software might not handle these files properly and the services that use these files may crash or shut down. A simple approach to testing this for an affected system is to remove the files responsible for managing the network interfaces on the system, which can be done by following these steps:
Ensure AirPort is turned off.
Go to the /Macintosh HD/Library/Preferences/SystemConfiguration/ directory in the Finder.
Locate and remove the following files in this directory:
Locate and remove any similarly named files with ".old" or "~orig" appended to their names.
After this is done, turn AirPort back on and try your connections. Do keep in mind that your network connections will need to be reconfigured again, so be sure to go to the Network system preferences and set up your AirPort, Ethernet, VPN, and other port configurations you may have.
General maintenance routines for Macs
General troubleshooting steps for OS X
In addition to the network configuration property lists, many times (especially after full system crashes), the problem is not that AirPort turns off, but that it seems to always be given a self-assigned IP address, even when connecting to a router that previously worked. In this case, in addition to removing the configuration files you might also remove the firewall configuration, since it may not be allowing the network interface to properly receive DHCP configurations (IP addresses, DNS servers, Search domains, and so on) from the router. The firewall configuration file is called com.apple.alf.plist and is located in the /Macintosh HD/Library/Preferences/ folder on the system, so remove it, restart the system, and then re-enable the firewall in the Security system preferences.
Generally when the firewall stops working properly you may see the system request that you allow or deny network access to a number of system services, instead of only prompting for applications you have opened; however, this will not always happen.