sounds to me like you've just disturbed the shift mechanism, no big deal. unscrew the gearknob and pull the trim from the gearshift surround. under there is a plastic cover held on by 4 x allen screws, think they're m6. anyway, remove them, you'll probably need a quarter drive socket set with some allen key bits for it as it's a bit awkward.
once you have the cover off if you look down the left hand side you'll see a 8mm allen bolt that attaches the shift arm to the selector rod, chances are this has moved. i'm taking a stab here but i suspect your lever is no longer standing straight up in the neutral position. you need to slacken this bolt and move the lever gently back to where you think it should be. basically what's happened is the alignment of the lever has moved in relation to the gearbox so now you're having to move the lever further to the left to hit first but before the gearbox selects you're hitting the detent for reverse.
Clive had exactly the same thing when he took his in for MOT except he couldn't get reverse. i'd guess that it'd be unlikely you've done the synchro on both gears, in any case even with a knackered synchro it should still go into gear with the engine turned off. i wouldn't worry too much at this moment, have a look first