Speedometer just quit all other gauges seem tbe working think its a fuse?

This is a pretty common issue for Trailblazers to have. Here's a link with more information about these GM gauge problems. And where you can purchase replacement parts on Amazon if you want to do the repair yourself.
Trailblazer Speedometer

I just noticed my speedometer is stuck @ 40 mph. It doesnt drop down when I shut down my vehicle? is this a sensor going out or what would be the recommended problem why it's doing this?

More likely is the sensor on the transmission either failed, or the wire came loose. You can check the wire by crawling under the vehicle, looking at the transmission, and you should see a sensor with a couple of wires coming off of it. Check the connector to see if it's secure. If so, it's probably the sensor. If it's loose, put it back on and check it again. Hope this helps you out.