While turning your Wlan on and off via hardware switch/key should work just fine on most laptops/netbooks, in some cases the poor Bios ACPI implementation creates conflicts between hard- and software keys.

First try to find an updated Bios, this might solve the issue.

Otherwise, install rfkill and use it as following after switching Wlan on via hardware switch:
rfkill unblock all
ifdown wlan0 && ifup wlan0

In this example wlan0 is my wlan device, replace it with the correct one.