My top eyelid is droopy. What can be the cause of it? Canit lead to something serious?

Ptosis. Blepharoptosis or just ptosis refers to excessive drooping (>1.5mm) of the eye lid below the limbus (outer edge of pupil). Aponeurosis (separation of lid muscle from insertion) is most common. Myogenic or neurogenic such as horner's syndrome (third nerve palsy) is also common. There are other causes as well. If this something new, consult your doc. Good luck.
Many reasons. It all depends on your physical exam and the history of your condition: bilateral or unilateral, just happened or was gradual, etc. Droopy eyelid can be congenital, due to nerve, muscle or combination.
Several explanations. There are several explanations for this problem. It is impossible to say without photos or an in-person examination. Droopy eyelids can result from drooping eyebrows, full upper eyelids or a true droop or "ptosis" of the upper eyelid. The treatment options depend on the exact problem. More importantly, if you do have a ptosis, it needs to be further investigated to rule out and underlying issues.
Need more specifics. We need to know more. How long, what caused it, other symptoms? Could be no big deal (chronic, benign) or it could be vision or life threatening (from infection behind the eye).