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!