Oh! This is an excellent question. Well, the answer that follows is technically just speculation. To truly know what’ll happen in the absence of nitrogen is impossible. You do mention that only nitrogen from the air has disappeared. So we will assume that other nitrogen-based compounds do exist. The nitrogen cycle will change.
Hypothetically, if nitrogen disappeared leaving a void with nothing to replace it, the earth’s atmosphere practically ceases to exist. This would directly be followed by a mass extinction of nitrogen-based bacteria and fungi. Following them will be the plants since they thrive on the roots’ ability to absorb nitrogen. Once the already existing nitrogen of the soil is gone, there is a shortage of nitrogen so the extinction of plants is inevitable. Although it will be a bit slower.
There is some leeway for plants because of what happens to animals and humans. Humans and animals choke and burn to death very soon. Nitrogen may be inert but it prevents the much more reactive oxygen from supporting combustion too much. Oxygen is actually toxic to humans and animals in large quantities. It has caused extinction level events in the past. We would all be poisoned to death. The poisoning means that as our bodies break down there is a little more nitrogen in the soil giving plants some more time.
Since oxygen is now abundant small fires will burn with more flames and will burn faster leading to a substantial increase in carbon dioxide and water vapor. This will start increasing global temperatures and the earth will literally start burning itself (Bye Bye, Plants!).
Aquatic life will probably survive since nitrogen is not as abundant in water although many nitrogen-based species will go extinct. Compounds of nitrogen that exist on Earth will react with oxygen to release nitrogen and some nitrogen will come back in the air.
If there were no nitrogen in our air, human, animals and plants would all die. Nitrogen comprises 78% of the earth’s atmosphere and it is critically important to all life on earth. Assuming the nitrogen in our air was replaced with oxygen, humans would die because our lungs would not be able to handle so much oxygen.
The air we currently breathe is only about 21% oxygen. With 100% oxygen at normal pressure, our lungs would fill with fluid, the alveoli (thin-walled air sacs in lungs) would not be able to exchange oxygen for carbon dioxide and it wouldn’t take too long for the person to die of acute oxygen poisoning.