Apple TV help...

My apple TV was working perfectly for a week, now i cannot locate the icon to 'mirror' any more at the top of my iMac, or send music to my tv/sound system. I have searched through 'system preferences'-'Display' and have home sharing 'on', I have restared both my Mac and Apple TV, i have double checked that both apple tv and my Mac are connected to the same wifi network. and have what i think are all the correct settings, but still wont work.. help!