screen went gray andscreen grey and appeared a folder with a flashing question mark inside

your Startup Disk's Operating System (OS) can not be found: That's what the "?" means.

If you have you OS installed on an external device you need to reconnect that device,

if you have your OS installed on an internal device (HD or SSD) something happened to the OS and you need to repair it.

If you do not have a copy of your OS 10.10 Media on USB or DVD available to you then you will likely need to reboot and hold down CMD+R, to go into Repair Mode and follow the prompts to re-download Yosemite. Do not format your Hard Drive or you will loose your data.

If the Hard Drive is damaged which is a possibility you will need to repair it or quite possibly replace it.

you may also be able to connect your mac to another mac using target mode and USB or Firewire cable. If so you can safely copy the files you need to a the other mac, or another device for long term or temporary storage so you can restore your OS without fear of loosing your data.

How to use and troubleshoot FireWire target disk mode - Apple Support

if that does not work I recommend you can try Disk Warrior 5 which is about $60 for a bootable flash version of an often indispensable application.

Might be of some assistance

Mac OS X: Gray screen appears during startup - Apple Support