I am posting this for a friend who has lost all access to two computers, a MacBook A1278 and a iMac 27inch, A1419 - both now running 10.10.4.

Friend is, and always has been an early adaptor, is way ahead of me, so I have no clue as to what he can do.

This is the message he sent to me

Both Machines are down.
1. Updated to 10.10.4 beginning of week.
2. Both Machines Networked and have been successfully file sharing (as usual)
3. Midweek had problems with Apple TV communicating / broadcasting "wirelessly" with iMac - so switched to Ethernet - which have done in the past.
4. ATV becomes worse to unwatchable.
5. Noted after research there was no update for ATV along with 10.10.4 and iOS 8.4 6. Really wanted to watch a show yesterday through ATV from iMac so decided to do a Disk Utilty permissions verification and ShutDown & restart on both iMac & ATV.
7. iMac would not re boot ATV "link behaving strangely"
8. Decided to move MacBook into another room and port it manually into TV - to discover it had become completely unresponsive and required a restart. - MacBook now in exactly same state - will not boot up - as the iMac.

Any help will be appreciated

Troubleshooting under these circumstances may be effective but will probably go very slowly. Can you just go over there and take a look? I would bet you find something unexpected and easy to fix.

My apology for not getting back to you sooner. I have been away and only just got an update on this problem when I back.
Several people had taken a look, Apple Care advised a trip to a Genius Bar - eventually they found a problem but were not clear on what had caused it - so I can't update you on that, but thanks for responding.
Both machines are now running smoothly, both have been updated to the latest version of Yosemite.
Kate