Nikon D3100 stops taking pictures long exposure?

I'm taking 30 second exposures of star trails with a nikon shutter wire release. It will take the 30 second pictures over and over but, I come back after 3 hours and the camera stopped taking pictures after about an hour. Is there a setting I can change to fix this?

Added (1). Battery works fine.

You would need to look at why it stopped. There are a number of possibilities such as the battery in the intervalometer no longer had sufficient charge to fire the shutter.

The most probable cause though was that used for extended periods the camera sensor begins to heat up. There will be a point at which the camera decides the sensor is too hot and so refuses to fire the shutter and probably puts the camera into a sleep mode. How subsequent requests to fire the shutter after that point are handled differently between camera models.

If that is what's happened I don't really know how you are going to get around this using that camera model, other than increase the time between shots. It may be interesting to look at the EXIF data for the precise time of the last few shots that were taken.

Memory full? Format before you begin. Time set to 1 hour? Lengthen. Sensor overheat? Stop if you don't want to break the camera.