Q: How can i get airplay to work

From what Mac or iOS device? Have you gone through these steps? What happened when you try?

Use AirPlay to wirelessly stream content from your iPhone, iPad, or iPod touch

About AirPlay Mirroring in OS X

Matt

try either turning air play off then on in setting or just turn your whole mac off then on again

Was it working prior to the OS upgrade?

What model (year) MBP?

Ensure ATV and Mac are both still connecting to the same network

If on wifi try ethernet

Try a reboot to ATV, if it persists reset.

Did you try the suggestions above?

You may want to get a network report to see it's current status, go to istumbler or similar and look for signal strength plus any noise detected.