The Blower Fan Motor Could Be Worn Out. They Usually Become Intermittent First Before Failing Completely.
Can you see where the blower housing is under the passenger side dashboard?
Is there a plastic panel there that doesn't allow you to see anything?
Either way, with the car stopped and engine running or engine stopped, but key in "on" position (where fan should operate) and fan control turned on, hit the panel or the blower housing with your palm. You may have to hit it several times.
The fan will probably turn on, indicationg that most likely it is time for a new blower motor / fan assembly. Most are not difficult to replace, just costly for the part.