hi all my astra 1.4 manual gear box got stuck in 4th gear to day

i could still move the gear stick in to 1st 2nd and reverse but not 3rd or 4th

i took the selector lump off the top of the box it all looked fine no broken bits

after pokeing around abit more i can now get all the gears again

my question is why did it happen ,will it happen again ,how and what do i replace to stop this happening again

i have to travel 120 miles a day so i cant risk just waiting for it happen again

thanks

wayne