how can i fix my ford transit gearbox?

ello all, been havin trouble for a while gettin my 54 plate transit into 2nd gear, eventually last week the whole thing went. seen my mechanic who removed gear box and replaced a pin in the gear box then refitted it but was having trouble in 2nd, 4th and reverse gears, he said it was the cables , replaced them, still the same! now been told its the gear stick, i assume this means the selector, HELP!