Notes are out of tune

I opened Musescore today and pressed play, but immediately noticed that the notes on the staff didn't match my piano. I tested them again against an electric piano to see if it was out of tune, but both my pianos matched each other and not the program. Everything in my score is played almost a half step lower than it should be, but not a full half step. When I tell it to play a C, it plays between a C and a B. How do I fix this?