I am getting zero displacements, pile reactions etc after analyzing a mat in STAAD.Foundation Advanced. What could be the reason ?

Please go to the Displacement Summary or Pile Reaction Summary tab within the Output section and see if you get non-zero values or not. If you get non-zero values there, but zero values for the current load case, then that particular load case is not included as part of the job and is hence not analyzed. You can check the current load case from Current Load combo box at the top. If you change the current load to any other load that was included as part of the job, you would see non zero results. To check which loads are included as part of the job, you may go to Job setup > Edit Current Job and look at the Selected Load Cases window on the right hand side. Any load which shows up inside the Selected Load Cases window is included as part of the job and is analyzed whereas loads showing up under the Available Load Cases window are not analyzed.