Turn it off, take off your lens and then remount it. If that doesnt work try the turn it off take out battery and put back in and see. I think the F error is the sensor points arent coming into contact with the lens.
Worth noting...I have problems every so often with my Tamron 180mm losing connection when it's still mounted.....it fit's slightly loose (not bad, but there's a little left to right play). Sometimes it will get to where if I move it with that play it loses contact.....lost precious macro shots due to this.
Fix is easy. I take the lens inside and remove it from the camera. I get a qtip and dip in rubbing alcohol and clean the contacts on the camera body and use the other side of the qtip to clean the contacts on the lens. Re-attach and it works perfectly. I just do this now at first sign of issues....planning to maybe pick up some of those individually wrapped alcohol wipes to keep in my camera bag.
Oh, and I will probably hit 10,000 shots in the next couple of days and mine is still going strong.
I was told this at the local photo store. take the lens off hold the body upside down (mirror down as to not get any dust in the camera) and rub the contacts with an eraser. Then clean the contacts on the lens.
I know my father in law had this problem from time to time too because he's never taken his lens off in the year he's had the camera. I however have the 35mm 1.8 and am constantly swapping and I don't have the problem anymore.