Hi Michealr
Try Control panel/sounds and audio devices. Hit the AUDIO tab and look for anything to do with MIDI. You should also be able to right click the speaker volume icon if you can find one (choose adjust audio properties.
Then try the drop down box in MIDI just in case something has changed it. This happened to me some time ago and I never found out what changed it, but afterwards all was OK. Try any settings available, if they don't work,put them back the way you found them.Good Luck, T.