Nikon FG shutter
By "stayed up", do you mean that the mirror remained in the up position so that you could not see anything in the viewfinder or do you mean that the shutter actually remained open? I do not remember the exact features on your camera, but in general, this is what I would do.
1) Remove the film from the camera and take a picture. Note if you see anything in the viewfinder. If you see nothing, the mirror is in the up position. Verify this by removing the lens and opening the door on the camera back. Look through the lens hole. If you can see through the rectangular hole at the film plane, the shutter is open. If instead you something is obstructing the hole, the shutter is closed. In any case DO NOT TOUCH THE SHUTTER! The shutter is what is covering the hole.
2) If the mirror is in the up position, check to see if your camera has a feature that allows locking the mirror in the up position. Some cameras have this feature to allow use of lenses with very short focal lengths. If this is the case, simply unlock the mirror.
3) If the shutter is the problem, your shutter speed may be set to "T", which stands for time. In the time exposure mode, you press the shutter release once to open the shutter and a second time to close the shutter. Cameras with a "T" setting also have a "B" setting, which stands for bulb. This is a throwback to the old days when it was common to use air-powered shutter releases rather then cable releases. The bulb was a rubber bulb that you squeezed to force air through a tube and push a pin to activate the shutter. The "B" setting keeps the shutter open as long as you hold the shutter release in, but as soon as you take your finger off the shutter release, the shutter closes. Both of these settings are used to make timed exposures. If you find that the problem was that the shutter was set to "T", set the shutter speed to 1/25th second or so and try again. The following sequence should occur; the mirror will flip up and the viewfinder image will disappear, the shutter will open for the prescribed time and close, the mirror will return to the down position and you will again see through the viewfinder.
If this does not work, you may need to take it in for repair.