sorry to hear your troubles, not good eh!
the gearboxes have a long history of 2nd gear synchro issues, but they normally tire out gradually, getting more nopisy and crunchy upon changing into 2nd, going either up or down the gears.
usually, when the trouble starts, you can drive around it by letting the rev's drop significantly before engaging 2nd, and then eventually it breaks altogether i expect, but to prevent engaging the gear without any crunches and warning first, is a new one on me.
was their no noises or misbehaviour at all prior to the failure?
either way, it sounds like 2nd gear synchro has shot its bolts to me, so a rebuild or replacement box will be required.
if it struggled to get any other gears, i might have suspected linkage, or indents, but from what you've said, 2nd synchros sounds like the culprit to me.