Computer won't boot up! A screen shows up with exclamation mark!!! What now?

shamsiii,

my guess would be that it’s trying to redownload your MacBook Pro’s original version of OS X (presumably Lion) from Apple’s servers, because it’s unable to detect a valid filesystem on your MacBook Pro — but that it isn’t able to download it because your wi-fi connection doesn’t support WPA or WPA2 security.

Are you currently able to boot into your recovery partition, by holding down ⌘R as your MacBook Pro starts up?

I had this same problem, maybe skipping Disk utility inadvertently: wouldn't go further than spinning gglobe and confirmation of Wifi connection until error message appeared After trying to reboot.

i Took the MBA over to my Dad's house, where there's VM fast cable, and connected to Wifi there: Bam, straight onto progress bar and less than an hour later I'm through DU with OSX Yosemite up, running and registered with just the Pages, iPhoto etc. Apps left to download.

I guess sometimes certain ISPs don't do well with Apple servers, for reasons unknown to me, and it was certainly worth a short journey to try another connection before writing off the machine or trying to do stuff with gigabit cables/adaptors or flash drives, which is beyond me!

Ps. I used Command +Alt after powering up to connect, not Command+R. Seemed to be easier to get to network connection that way.