What is the cause of constant headache?

Dehydration/Tension: Usually dehydration and lack of sleep or stress. Ensure you're taking enough fluids. Try getting a good nights sleep. Stay away from bright lights and loud noises. If these don't cure the headache ,try taking some Tylenol or Anti inflammatories. If none of these help then you need to see a Dr to investigate the cause of the headache

Many: depends on the type of headache: ex-migraine vs tension vs due to a brain tumor vs due to sleep apnea vs high or low csf pressure headache etc, each separate headache type has its own cause. consider speaking to a neurologist with headache expertise.