It sounds like your MIDI device is not sending data to Studio One. You will want to go into Studio One>Options (Preferences on a Mac)>External Devices. Locate your MIDI device, double-click it and check the box to make it your default instrument. If you have already started a song, press the letter "i" at the top left-hand corner of the screen to view the Inspector window for your track. Make sure the input is set to the MIDI device that you wish to use. You will also want to open the MIDI Monitor by clicking the icon of the 5-pin MIDI cable at the bottom left-hand corner of the screen. With this window open, press some keys on your MIDI controller and verify that you are receiving MIDI data. If you are not, check your MIDI ports in the External Device list. If the problem persists, please try uninstalling/re-installing Studio One. You will then want to try removing your settings files from the program and re-configure your MIDI device. Here are instructions for removing your settings files.
1. Go to Help>Open Settings folder.
2. Copy the contents of the folder over to a folder on your desktop.
3. Close S1 and delete the contents of the settings folder with the exception of the user.license file as well as the "Extensions" folder.
4. Open S1 and go to Studio One>Options (Preferences on a Mac) to reconfigure your settings.
5. Test to see if the problem is resolved. If it is not, please feel free to copy your settings files back over from the backup that you created in step 2.
*If you are missing presets or Sound Sets after doing this, simply double-click one of your Sound Sets under Documents>Studio One>Sound Sets.