Hi,
It sounds like possibly a MIDI driver issue. You can try updating your MIDI drivers. You can also use the "Turn off stuck notes" (at the bottom of the "Perform" menu) to turn off notes that are stuck on, so they'll stop playing.
If you're using the old MidiNotate and you're on a Vista machine, there may be other issues as well. With the Vista operating system, the way that MIDI and audio are handled changed tremendously. The result is that some major updating had to be done to MIDI and audio apps, and for us this resulted in the release of the 2.0 Notation series of our applications, which handle those problems. You can try out either of the new products free for 30 days to see if they address any other audio issues you may have (
www.notation.com/Download.htm) If so and you want to upgrade, there is a price break for previous customers (see
www.notation.com/UpgradeOrder.php).
Hopefully something above will help you get going
Sherry