There are two possible problems with the calculator. First is the screen's contrast may be set too low for anything to show up. The solution to fix that can be found here:
[[Casio FX-115MS Troubleshoothing#Section_Low_Contrast]]
Secondly the batteries may be dead and need to be replaced. The answer for which is here:
[[Casio FX-115MS Troubleshoothing#Section_Dead_Batteries]]