Help with air conditioning diagnosis

Hi,

Can anyone help me with the following problems with
air conditioning? The fan in the air conditioner unit outside
the house still rotates, however with the slower speed.
The temperature in the room doesn't decrease. I notice
that there isn't any ventilation noise. But when i put
my hand close to the ventilation hole, i can still feel
air blowing out, however much weaker and not quite cold.
The sympton is the same when i set fan at either auto or
on position. Thank you very much for your help!