What program, or command were you using to activate wlan0? Is wlan0 listed in /proc/net/dev?
The error message would be more useful if it indicated which File or Directory was missing. That
might indicate whether you don't have a kernel module loaded, or are not running wpa_supplicant.
Look at "sudo lspci -v". Near of the bottom for the section on your wireless device, it lists
the kernel module used. Is that module loaded?
Look at "sudo ifconfig". Is wlan0 listed?
Look at "sudo iwconfig". Is wlan0 listed?
Run "iwlist wlan0 scan". Does it list detected access points?
Run "ps -u root | grep wpa". Is wpa_supplicant running?
Try "service network restart". This should run ifdown & ifup on all of your network devices. Where any errors printed?
If your wireless device sees other access points, sometimes, your best bet is to start over, removing the device in system-config-network & system-control-network. One of the config files like ifcfg-wlan0 or ifcfg-Auto-<yourssid> may contain an error which the networking scripts choke on. Add them again after afterwards.