How to show vector features in OpenLayers after hiding it?

I did change some vector features style (through check-boxes) using style property :

var features = layer.features;

for( var i = 0; i < features.length; i++ ) {
  //features[i].style = { visibility: 'hidden' };
    features[i].style = 'none'; 
}

layer.redraw();

But now if I check the box again, it supposed to display again but nothing happens! I tried:

     features[i].style = 'block'; 
OR
     features[i].style = 'delete'; 

then redraw the layer.. but this doesn't work

Any Idea ?