The recoater roller has probably stopped rotating. In order to fix this, apply two drops of oil to each of the recoater bearings (at both sides). Another reason that may have caused this: the recoater transmission rope (responsible for its rotation) may have broken. This is easy to ascertain as one of the strands that is always located by the recoater will simply be broken in such case. This will be visible to the naked eye, in which case replace the strand.