Autofocus on video for nikon d3100 won't work

Okay, this is probably my own doing, since I've been messing a little with the settings to shoot on manual, but now I'm filming videos, and I need it to auto focus, but it won't. Can someone tell me what exactly I should be looking in?

There should be a switch on the side of the lens. Is this set to autofocus?

I'm pretty sure the D3100 has AF in video mode. As already stated, check the lens is not switched to manual focus and then consult your user guide.

Press the <i> button and navigate to the <m> in the column on the right and change it to <af-s> or <af-c> or <af-a>
You know what i mean if you own a lower end nikon.
answer mine?