When a resource-intensive task is processing, such as a complex formula, macro or query, it can cause Excel to become unresponsive.
This is particularly annoying if you have multiple workbooks open, as you are forced to wait because you cannot switch to the others.
The solution is to open a new instance of Excel. This window will act independently of any other.
Simply hold the Alt key whilst left-clicking on the Excel icon in the Start menu.
A window will pop up asking you if you want to start a new instance. Click Yes and a fresh window will load.