Ptosis. A drooping eyelid is called ptosis and can be caused by a wide number of things. If this is a new onset problem, you should see your eye doctor soon in order to determine the cause of the problem.
Many causes. Drooping eyelids can be caused by aging and drooping eyelid skin, sagging eyebrows, stretching of the levator muscle ( lifting muscle of eyelids) or more serious systemic problems. Consult your ophthalmologist to determine the cause then choose an oculoplastic surgeon for the correction.
If this is new, You should have it evaluated. There are multiple possible causes. Start with your family doctor, and he/she can see whether you need a referral to an ophthalmologist.