hmm could be the lens,could be the sensor
if its the lens,use a blower to blow the lens then use lens cleaning cloth to clean it
If its the sensor,for canon,activate the mirror lockup and use blower to blow thw sensor
if not,bring to csc
if u had never done it before, suggest that you send it back to the service center...spend some $$ but at least you wont accidentally scratch your sensor and spend more $$$ repairing it.