I have an Acer aspire with built in 3G (Umts) modem. After changing the SIM, it gives me a message "The Radio is Off" . I assume that meant by that is the radio transmitter that sends the signals to the nearest umts station.
The hardware radio control switch is the manual switch that would turn on or off your wireless devices.
With your laptop it should be in the front panel of the base, the part that is facing you.
The first one is for your bluetooth and the second one is for your wireless card. You should be able to slide that and turn it back on.