Thanks. As it turns out, I must have done something wrong in 'loadout' as has been suggested. I went back in and 'clicked' on this and that, and changed the default setting etc for the guns. At one point neither my MGs or cannon were working!! So I resorted to my usual approach and just started feverishly changing things at random and in an ever more panic stricken kind of way
. Eventually my persistence paid off and both weapons sets re-activated. I still have no idea why
.