This is an unusual experience that is happening to some TurboTax users. If you are using a desktop version of TurboTax, there is a workaround.
You can enter your Social Security income using Forms mode. Then, delete the entry and revisit the Social Security section in the step-by-step process. That seems to reset the pages back to normal.
To do this, use these steps:
Please let us know if this does not work to allow you to enter your Social Security income.
Also, if you are a TurboTax Online user please reply back with that which version you are using.