How old of a copy of the driver/software are you using?
In all of the versions I've used over the past 3 years, you can set the table height under the screen on the second (or third?) tab. If it's reading negative, I suspect the machine won't allow you to move it any more. It should know what lens in in there, and then you'll want to set the table to 0.000" using their little focus spacing tool.
I've had mine mysteriously lose that setting, and I've also had a push-button control panel fail (replaced under warranty) - both cause similar issues.
My ULS machine is being supplemented with an imported machine that should arrive on Monday - 48x60 @ 150 watts.