Mac is stuck on startup after updating to OS X Yosemite 10.10.5

I have just updated my Mac Air to OS Yosemite 10.10.5 and can't get it to start again, it gets stuck halfway the start up. I have tried Safe Boot but not able to. I tried Recovery Mode, verified disks and all seems ok. I tried re-installing OX 10.10 but not successful. What should I do? Thanks in advance.