You need to post an image and camera & lens settings of the result. Blur can come from focus, aperture setting, camera vibration, and using vibration reduction.

For focusing, did you try the optical viewfinder? This works with long lenses (> 200 mm). However, you should be able to use live view with a longer lens. In live view, you can magnify the image which helps resolving stars. Also, see if you can find a bright star to focus on and lock your focus ring using a piece of tape.

Wide angle lenses (< 35 mm) are difficult to focus on stars. Some wide angle lenses I own have a distance scale on the lens. I auto-focus at some object far away during the day or the moon at night and note where on the focus scale true infinity is. The infinity marking on modern lenses is not precise.
Lenses that don't have a distance scale, I'll count knurls on the focus ring from true infinity to the focus ring stop (clutch engagement) near infinity. When auroras show up unexpectedly on camping trips I can easily set my focus distance to infinity in the dark (red-light headlamp assisted).
The easiest solution is to set your focus during the day. Once focused, use tape (blue painter's masking tape) to lock the focus ring to prevent accidental movement of the focus ring.

Camera shake during long exposures can also cause a "blurry" image. Unless you have a very stout tripod, keep the tripod legs length at minimum. I made a foam brace for the center column on my tripod since the center column is significant source of vibration issues.

If possible, don't shoot wide open. Even 2/3 stops from wide open will help with image sharpness.

Use a remote shutter release. The D7100 can use the ML-L3 IR remote which is very inexpensive. If you don't have a remote, use the 10 second self timer delay feature which allows tripod vibrations to die down.

Be sure vibration reduction (VR) is turned OFF. VR systems drift with long exposures (> 0.1 seconds). This is evidenced by star streaks, sometimes difficult to separate from Earth's rotation issues.

If you don't have a tracker (device that rotates the camera to match Earth's rotation), you need to limit the shutter speed to something reasonable to minimize star trails. My general rule of thumb is 15 seconds for a 20 mm lens on an APS-C body. I will scale the shutter speed depending on the focal length of the lens. Example: < 1.5 seconds shutter speed for a 200 mm lens.