Sorry to hear about the recent issues you have been experiencing.
You will need to go to Options > Audio Setup and choose an audio device that works on your system. Without an device set here, neither audio nor MIDI will function in Studio One.
This may be something that requires a bit more troubleshooting. Please create a support ticket if you would like to discuss this in more detail.
Here is a link to create a ticket: http://support.presonus.com/hc/en-us/requests/new