Hi this is actually extremely annoying, I’ve been using blender since late 2009 and I know how to get around in it, so to speak, but as soon as I downloaded v.2.60, I’ve been getting this problem: I’ll be modelling something, and when I go to render it to test it’s progress with lighting and such, the model disappears. I’ve tried changing materials, playing with the material settings, and everything is on one layer. It’s not a consistent thing, but when it happens the only way to fix it is to join it to another mesh and separate them later. The weird thing is, it’ll render just fine once, then it’ll disappear later on. It’s enabled for rendering in the outliner and everything, and it’ll just be default settings when I first open blender. I have no idea if this has happened to anyone else, but it’d be nice if someone could shed some light on the matter 
Thanks, Boonze
I think we would have to see an example BLEND file to debug that.