Two seperate and distinct offences. 1) Failing to have and 2) Failing to display. Obviously you cannot display what you do not have (1) but if its fallen off and lying face down on the floor, then the TW only sees the holder and cannot see the excise licence, and so gives you a £30 badge (2). Likewise, if its slipped down the screen a bit and the regno is hidden but the expiry date is still visible, again you get a £30 badge as they cannot say if its your disc or someone elses - potential fraudulent display.
Otherwise, its as Ron McB states - lost, stolen, whatever and a conscience call to you. 
Its a £60 fine in avon and somerset force now... as is no MOT!
If i can remeber to go to the post office or click my mouse to get a new tax disc then so can everybody else!... If its fallen down then get a proper holder.. not the metal ones from NEXT that dont stick to the windows and people rest them onthe dash!... i hate that! nothing worse than me tearing after a car because it has no tax displayed to find it onthe dash because the lazy *** cant spend 50 pence on a proper holder from the local shop or post ofice! its all about PRIDE!
If they have made a real effort to fix it in a proper holder and its accidentely fell off = no ticket
If its a crappy holder or masking tape..or trying to defeat the laws of gravity or just plain forgot to get one then = TICKET....
While i am here... why have you got no tax? is that because no insurance? no licence? disqualified? etc....