Error 40 : Battery related error:
Clean the contacts of both battery and camera with isopropyl alcohol. Try replacing the battery.
To get rid of the error temporarily in order to use the camera follow the procedure, switch on your 7D and switch off the moment you see the error, remove the battery wait for atleast 30 secs before putting the battery turn the dial to movie mode, put back in the battery and switch it on the error will go away now quickly go to manual option and then menu and disable lens cleaning option the error will go away but will come back after you switch off, so you'll have to repeat the step.