I went out for a quick blast the other day in my new toy (Astra GTE 16v) and it went into every gear fine. After I dropped some mates off I could not select 2nd of 4th gear. I'm hoping (as Cemesis said) that it is just the gear linkage needing adjusting?
I've heard something like this has happened to 0ddball?
checking the alignment is the second step, haynes manual is your first step. In there it describes the readjustment brilliantly, which basically involves pulling a long ribbed plug out the top of the gearbox selector mechanism. twisting the selector rod anti clockwise and then sliding a 4.5mm drill down the hole which locks into another hole on the internal linkage. then slip up the gear stick gaitor and there is a pointer on the left of the stick carcass that should line up with a brass marker on the stick outter assembly, just where the fork lifts up and over for reverse.
If its correct, with the drill in place, the gear stick should be in the neutral poistion but held over as far left as it goes, touching the cream plastic gear stick box surround.
If its not right, then there is a pinch bolt that joins the gear selector rod to the shifting mechanism in the engine bay, and that is just the other side of the bulkhead. slacken, move gearstick to above position, then tighten and pray.
If that dont work then your gearbox is partially shagged, just like mine, welcome to the club.