I'm using X3 and I've done this a million times but I cant work out what has happened .I'm editing a piano track and adding notes and pedals etc.So I'm recording over a track that's already there ,but Sonar wont play the track when i hit record, it's just silence.All the other tracks play and if I stop recording and just hit play they all play fine.But when i select record on this track and hit record all the other tracks play and what has been previously recorded on this track is just silent.What have I done ???????

Could it be the recording mode? IIRC If Overwrite is selected, then it's supposed to be silent 'cause it's overwriting what's already there.

Yes, it's likely the record mode. Right-click the record button in the transport module and choose Sound on Sound mode.