What happens if my machine crashes and I want to use the ETS Question Bank on another machine?

The license is not tied to any machine. It is tied to a customer. So you can use the same License Key on any machine you want to. If you have lost the License Key and are within the support period, you can supply the Orderid and Emailid in the automatic license tab (this is displayed when you open an ETS Question Bank) in ETS Viewer to let it download the license automatically.