Assuming 8v and rod change - get yourself a setting tool from the bay for about a fiver:
Remove gaiter, fit tool, slacken allen bolt and rotate until correct play: it's possible yours has worn/slipped and it's preventing the lever from moving the correct throw to select fifth.
You need to establish though if it's physically not moving far enough right to get 5th (ie is it actually any different with engine off).
I'd also have a look around the linkages under the bonnet onto the back of the gearbox and check everything is connected, you'll see a couple of white plastic links snapped onto ball joints at each end (look like dog bones). Make sure they're properly fitted and not obviously worn.
