Instead of seeing the Finder or the login window after startup, your computer may stop responding and the display may only show a persistent gray screen (you may also see an Apple logo, spinning gear, spinning globe, prohibitory sign, or a folder with a flashing question mark on it).
This usually means it cannot find an operating system. Boot from your original System Install DVD or OS X Recovery, depending on the version of Mac OS X you have.
My MacBook Pro fans were loud and unusual, so when I looked up what to do on google most of the results told me to reset the SMC.
This is usually the remedy for out of control fans.
So I did that, expecting it to work, and now my MacBook Pro won't go past the Reboot screen! The strange thing is, it doesn't stay on the Apple logo like most people say. Instead, the apple logo changes into the "Restricted" Symbol and just stays there.
Ok, it's not booting up to OS X, which when loaded will control the fans to settle down, but the default if OS X doesn't load is eventually turbo mode.
You have basically what's called a "gray screen issue"
Go through the Steps here and it will fix it,, provided it's not a hardware issue.