If Solver gives you an error message box when you try to get the Answer or Sensitivity reports, remove the Solver Add-in and then put it back. Here is how:

Click the Office Button.  (It's circular, at the upper left corner of the Excel window.)
Click the Excel Options button.  (Rectangular.  Near the bottom of the pop-up window.)
Click on Add-Ins.  (In the list on the left.)
See that next to Manage: it says Excel Add-ins.  (If it doesn't say that, click on the little box and fix it.)
Click Go...
Uncheck Solver Add-in.
Click OK.
Solver should be gone from the menu.
Repeat the above, except that this time check Solver-Add-in.
Solver should be back on the menu.

You may have to do this every time you use Solver on a new file. Sorry!