That would be the least of my worries if someone had taken control of my PC. I'd want to stay offline until I'd found out what he'd been doing.
That said, he might have literally disabled it. Go into Control Panel>System>Hardware>Device Manager and expand Network Adapters. Right click on the Wireless device and go to Properties. If it's disabled, reverse that change and OK your way out.
After about a minute it should come back on.