assuming none of the chain from the light switch is visible outside the light fixture on the fan. depending on the type of fan/light fixture you have you will need to turn off power to the fan/light, then get into the light fixture and remove the light control switch. rememember which wires are connected to which on the fan and light fixture switch. assuming its a simple on/off pull chain switch. take it to Home Depot or lowes and get a replacement part. and reverse the removal procedures to re-install. However if part of the chain is still visible on the existing switch. try and tie a nylon string to the end of the chain and try and operate the light switch off and on to see that it is still working. replace everything and cut nylon pull string to proper lenght. good luck