well one of the worst things one can do is 'fiddle' with a lifttruck, especially if one does not know what they are doing :o/

if display is not displaying any codes or has any indicator lights on for the drive system then all we can do is guess what is wrong.

Generally when the BDI gets low the lifttruck will slow itself down to save power to allow you to make it to the charging station to recharge the battery. This is what it sounds like happened but after recharging the battery it did not reset when the fully charged battery was re-plugged back into the machine.
Question: the BDI did reset back to full correct?

If it did reset and the lift is still driving in 'limp mode' then it sounds like you need a service tech to come and take a look at what your other guy MIGHT have accidentally changed IF he did happen to gain access to the service menu.