What format are the files in? And do you know if they currently play outside of Studio One? It might be an issue that they have since become corrupted somehow and Studio One can't physically play them. Or perhaps some sort of configuration issue? If you have studio one professional it comes with a few demo projects. Try to see if those will play. If not, just download an .mp3 or .wav file from the internet to see if anything happens.
The last resort would be to reinstall studio one.