Blood clots usually form when you injure yourself. The blood clots and forms a scab, keeping the injured area protected until the body can repair itself. When a clot forms inside a vein, though, it can have an extremely negative effect. These clots can narrow or even completely constrict the flow of blood, which can lead to heart attacks and other negative side effects. When blood clots in the neck however, the risk is even worse because the neck holds the veins and arteries that supply blood to the brain.
One of the most noticeable symptoms of a blood clot in the neck is swelling. A blood clot might only constrict the vein or artery (clots can form in both places) partially rather than fully. A partially closed passageway will still be able to let blood pass through it, but that passage will be more difficult. The pressure added by the clot will cause the area to swell because your body recognises that there's an injury there that needs to be fixed--even if it's your body's natural healing processes that are causing the trouble.
If a vein or artery is completely blocked by a blood clot in the neck, a stroke might be another, potentially devastating symptom. When a clot in the neck stops the flow of blood to the brain, brain cells begin to die. Symptoms can range from weakness and dizziness to paralysis or the inability to speak. If the clot doesn't let blood through, or if the clot isn't removed in time, a stroke can kill the person suffering from it. According to the National Stroke Association, strokes are the third leading cause of death in America and the leading cause of adult disabilities.
One of the major symptoms of a blood clot in the neck is that there will be pain in the affected area. Your blood is being squeezed into a smaller area than it should, which can cause soreness and inexplicable bursts of pain. The area is often red and tender. These symptoms don't necessarily mean there's a blood clot in your neck, but they are suggestive. If you experience them, you should likely see a doctor.