Welcome To All
I purchase RouterBoard (RB951Ui-2HnD) , Mikrotik version 6.18 , when upgrade to 6.34 is not responding .
When you reset factory responds . and when work restarts due the same problem .
Please help . 
Do a netinstall: http://wiki.mikrotik.com/wiki/Manual:Netinstall
pukkita Thanks for the reply, but the problem still exists, when restart routerboard due the same problem .
What does exactly happen when “not responding”? You aren’t able to connect to it via winbox until you reset it again?
Can you connect to it via winbox?
If so, go to System > Routerboard and check if Current firmware is the same as Upgrade firmware, if not, click on Upgrade button. Reboot. Does it respond now?
If it doesn’t, System > Routerboard , click on Settings button, tick “Force backup booter”, reboot it and check if that fixes it.
Is it a brand new unit?
When reset factory and restart routerboard i can’t explorer with winbox and all ehthernet lamp not flicker !
(( If it doesn’t, System > Routerboard , click on Settings button, tick “Force backup booter”, reboot it and check if that fixes it. ))
ok now mikrotik is responding …
Thank you very match
Seems your main booter is damaged.
If it was a brand new unit, that’s covered under warranty, you can contact your seller for a replacement.