the fan on my macbook won't work

Run the Apple Hardware Test to see if it spots any hardware issues with your fan. Thats one of the tests it runs. When prompted, choose the extended test, which may take over an hour to run.

Assuming no sensor or other hardware issues were identified by the hardware test, have you reset your SMC?

Download, install and run iStat Pro to check temperature and fan speeds.

https://www.apple.com/downloads/dashboard/status/istatpro.html