The locking mechanism controlled by your break pedal has failed.
Call a shop
If the car will go into second gear and take off, then there might be a bad syncro gear or bend shifter fork. The transmission may need to be taken out and inspected to find the problem.
Check if the clutch is working properly.
Gear up then
1999 ford explorer 4.0 manual transmission gear shifter wont fit on shaft
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Your clutch is not disengaging.
synchro plates went out
my 94 Toyota corolla will not go into any gears .
The clutch is bad or out of adjustment.
bad transmission..... expensive repair.
Most likely a broken gear cluster
how does the belts go on a 1996 Mazda millenia