My iPhone 5 says there's 'No Sim Installed'

So, basically the problem is that I have a sim installed but my phone says there is no sim. I was on a call literally not ten minutes ago. And then I hung up, pressed the power button to turn it off and then turned it back on to take another call only to be greeted with the message 'No Sim Installed'.

I took the sim out, restarted a few times and all that. Any clue on what's happening?