Hello,
What kind of shifter mechanism do you have? The older style with separated linkage for drive and reverse?
First some questions, because if it had worked before, it should work afterwards:
It is a five speed, right?
Is the clutch working correctly?
Is the clutch lever installed correctly and not worn?
Is the clutch system bleeded correctly?
Is the front engine mount replaced and installed correctly?
Are the clutch cables in right position – as before – very important?
In general: if something is wrong with the length of the cable, that won't affect only 1 gear.
Regards, Markus