A painless dilated pupil which does not react can be a serious problem. Most worrisome is aneurysm, or dilation of the blood vessels in your head which is compressing the nerve that goes to your pupil. This needs to evaluated by imaging studies and needs immediate attention!
Assuming that you are young and healthy and have not sustained any injury to the eye or head and you do not have an aneurysm, the other diagnosis could be Adie's tonic pupil. This condition primarily affects women from 20-40 years of age and is considered benign.
Adie's pupil is a condition of unknown origin with asymmetric and inequality in the size of the pupils of the eyes. It is believed to be a result of damage to the parasympathetic nerve which innervates a muscle of the eye known as the ciliary body. . Accommodation, or the adjustment of the eye for distance, is affected, as well as pupillary dilation and contraction, the ability of the eye's iris to open or close in response to light. An Adie's pupil can be diagnosed by your ophthalmologist with specific eye drops- pharmacological testing.
I hope that this information has helped you but I HIGHLY suggest that you be evaluated for an cerebral aneurysm by the Emergency Room!