Midi controller not working

I am using SX3 through XP pro SP2 with a 512 RAM laptop. I have been using my M-Audio Axiom 49 to control VSTs. This has worked okay. However, it has suddenly stopped working. The VSTs are playing okay, e.g. I can play and hear notes played on the ‘B4’ keyboard, but no sound comes from the Axiom. Tried another midi keyboard that worked previously, but just the same. It would seem that some setting has changed but I can’t find out what. There is a signal showing in the mixer as I play the Axiom so the problem looks likely to be in the output somewhere. Can anyone help as I’m a bit stumped.