While driving along a major highway, I came to stop at a traffic light, with all internals on my (highly unreliable and generally dodgy-as-hell) 2000 hyundai excel seemingly running OK. Upon the light turning green and accelerating away from said lights, I shifted from 2nd up and across into 3rd gear at which point the gear stick suddenly gave way, failed to lock into gear and kind of limply flopped about. Long story short, I can depress the clutch entirely without the faintest hint of any issues (i.e. the problem doesn't seem to be clutch related), however I am unable to put the gear stick into any of the gears, and besides which I cannot even physically move it left or right in order to access the 1st, 2nd, 5th or Reverse gears. It pretty much just flops limply back and forth between 3rd and 4th (regardless of whether the clutch is in or not). Also, the car stalls unless the clutch is in (or unless I hyper-rev it enough to get it up to about 3rd/4th gear speeds in which case it seems to run quite happily). So... I was basically wondering if any of you experts out there could perhaps give me some idea of what was wrong, and how many limbs it's going to cost me to fix it? Is it something simple like a shift cable break, or potentially far worse?

Cheers in advance for any help you can give.

Sounds to me like your shift linkage has let go. As far as cost, I can't even guess.

Sounds like a linkage issue. Place the front end on jack stands and examine both linkages. Look at the linkages coming from the gear shift and pull back the boot and examine the linkages going into the transmission. There's often an 8mm pin that you will see inside the boot--make sure, for instance, that you see it. The other linkage often attaches to a bracket with a 10mm bolt. Basically, use your common sense and check things out.