Occasionally if you are running tests and you go to the Current Runs section of the Execution tab, you cannot see any tests in the Assigned Test Definitions section. This can also occur in the Activites tab, under the Current Executions section.
As you can see from the screenshot the Status heading text is in italics. This means that a filter is present on this heading.
If you click on the dropdown arrow for the "Status" column heading - look at the "Filter" selection in the popup menu, you will see the checkbox beside it has been checked. Uncheck this, and the assigned test definitions will appear in the whitespace again.