there are a few quick solutions that may fix your problem. First check that your lens is properly mounted and there is no dirt or anything preventing the lens from seating properly. If that doesn't work check the settings on your camera. The autofocus and speed may be up too high. Try to change both settings and restart your camera. There are a few more things you could try. For further instructions visit the trouble shooting page: [[Nikon D90 Troubleshooting]]