When taking photos with Nikon nikon d3100, why is there a delay of 30 seconds before shooting? How to adjust it back to normal?

In video mode: the camera will count down 30 seconds before automatically ending (not artificially ending) to remind: the camera video is going to stop. The reason for automatic stopping is that the camera cannot stop it. Just keep shooting, so the camera will automatically stop based on the storage space of the SD card, but there will be a 30S countdown prompt before stopping. Of course, there are other situations that cause the camera to automatically stop shooting, but it will count down before stopping. For details, see page 46 of the D3100 manual.