One of the synths on the track I am working on has lost the link to its midi track.

The volume on both the synth and midi channel is effecting output to the master but normally the volume slider on the midi channel doesnt do anything when the slider is moved. 

The volume slider on the synth usually overrides the midi channel strip and sets the master volume.

Not sure what Ive clicked but I need to sort it as the pro channel on the synths channel strip is now not doing anything when the modules are switched on.

Ive double checked the imputs and outputs of both channels and they are set up the same as the other synths in the song that are working normally.

Any ideas folks?

make sure the fx button on the transport is not green.  if it is, press e. that is master fx bypass.  the volume slider on midi channels sends controller #7 info to the synth. the synth must adhere to gm guidelines to accept volume input from controller 7 - some synths do not.  
what do you mean the volume is effecting the master??

I just mean the master volume of the synth but I will check  your suggestion now :) Thank you.

If the midi track lost it's connection to the synth..... simple go in a reconnect it. expand the track inspector  for the midi track output.... click it and the synth should be in the list.....select it and you should be back in business. You might need to select "ALL" to display all the features in the track inspector boxes.