BioMart does not return a result
Whenever you are wondering about things like this, just look up the identifier on the Ensembl web page. If you look up ENSMUSG00000083840, you will see:


  This identifier is not in the current EnsEMBL database


In this case, it doesn't seem to have been replaced by anything and it wasn't a protein coding gene, so it looks like it was simply a wrong annotation which has been removed.