In The Unlikely Event...

1. Pained innocence should always be your starting point, so begin by looking shocked and concerned, and asking loudly what on earth has happened.   If you're the one who starts making a fuss, everyone will assume that it's someone else's fault.
2. Check your bells - surreptitiously, of course.  To do it openly is tantamount to admitting that you could possibly be the guilty party - and we don't want to do that, do we?  So say something like "I thought it started to go wrong around Bar 48, didn't you?"  Then while everyone is busy looking for Bar 48, have a quick butcher's at the old handles, and quietly make any necessary adjustments.
3. If you are now sure that your bells are OK, it is safe to say "Has anyone got the wrong bell, by any chance?"  With luck, someone else will have, and the moment will pass amidst much laughter, backchat or stony silence and grinding of teeth, depending on the importance of the rehearsal. 
4. If no iffy bells are found, you need to go on to page-checking.  It could be that someone has failed to turn over (like at practice the other night when a certain nameless party put down her bell to turn over, grasped the corner of the page, let it go and picked up her bell again - then wondered where the music had gone.  It's called Sudden Attention Deficit Syndrome - or having the SADS).  
5. Or perhaps someone has taken up a favourite occupation of ringers - turning over two pages.  A clue to this is when the conductor / leader says, in the midst of the Wonky Bit, "Bar 31 - Now! 2,3,4" and you are on Bar 65.  If you're not fortunate enough to get such an indication, you should quickly look at the bar numbers on your neighbour's music and make any necessary adjustments (pretend to be looking back at an earlier bit while you flip the pages - oh, come on, I don't need to tell you that, do I?  It's a basic survival tactic which you should have learnt in the first two weeks!) .  
6. Or you may have failed to follow the required page-turning pattern - start on the second page, turn back, then forward two, then to the end.  Oh, sorry, forgot to say - there's the repeat on Page 2 that continues on to Page 1, so you'll need to go back, forward, back again then forward two pages.  Got that?  Good.
7. If it's neither bells nor pages, something more drastic has gone wrong.  Ask the conductor which piece he / she thinks you're playing.  Do a quick headcount to see if anyone has sloped off to watch the semi-final, leaving their next-door neighbour to cover their bells.  Is anyone on their mobile phone, or eating (crisps and dips are a real challenge in a fast piece, aren't they?).  The more outrageous the suggestion you can make, the less likely it is that the finger will be pointed at you - so you can quietly put away your knitting and bring yourself back into the swing of things!