....I took the lens off and it looks like the mirror is getting stuck in the "up" position and once I hit the shutter release again it then goes down but no picture is being taken....
.... if any other models other than the D90 roll over back to 1 after hitting the 10 000 actuation mark?....
It sounds like the shutter is set to "BULB" or a timer mode. In bulb mode, you would see the image sensor when the mirror is up, otherwise you would see the shutter curtains.
None of Nikon's cameras roll the shutter actuation count back to one. That is the
file numbering count, and it doesn't actually rollback to 1.
The only way to read the total shutter count is to make a JPEG image and then use an EXIF reader to find the line in the EXIF data that shows shutter count.