Did you replace it with a fan that's 100% compatible with your mainboard?
The above discussion about not disconnecting the battery before doing the replacement could also have caused the problem.
Another possibility is that you may have accidentally knocked off some small component(s) from the mainboard when you were prying open the laptop. Happens when the object you use to pry it open slips into the components near the edge of the mainboard. Knocking off even a single capacitor or diode can cause problems.