Q: AirPlay button missing in iTunes on iMac, how to fix it?

iTunes was updated on my iMac maybe 2 months ago and since then the AirPlay button in iTunes is missing. Before the update, AirPlay was working fine on my computer. I have a 27 inches iMac, mid-2011 running  OS X Yosemite (version 10.10.1) and iTunes is running on version 12.0.1.26. How can I fix this problem?

Thanks,