There are two things that could be wrong with your calculator.
First, one of the buttons may be broken and this is discussed on the troubleshooting page. If this is the problem, there is a link to a guide that will give you step by step instructions on how to fix that.
[[Casio FX-115MS Troubleshoothing#Section_Broken_Button]]
Second, a part of the keyboard may be broken or not responding and this is discussed on the troubleshooting page. If this is the problem, there is a link to a guide that will give you step by step instructions on how to fix that.
[[Casio FX-115MS Troubleshoothing#Section_Broken_Keyboard]]