Reinstall isn't necessary. Right-click (Mac Cmd-click) on one of the other panel headings such as tone curve, and make sure there is a checkmark in Basic. This seems to be a common phenomenon with some users. But I don't know what has caused it.
You can always download the LR version 5.6 from the update page from Adobe:
There is also a possibility that the Panel is hidden, When you right click on the Develop Module on the Right side,
You can enable the panels. Basic is the Panel where the Exposure options should be reflected.
When this doesn't work, you can try creating a New Catalog, before you completly try to reinstall the application.
Hope this helps.
thanks gang rt click did it