If the following doesn't help, then take it in for service because you may have a hardware problem based on what you described about the screen.
Reinstalling Lion/Mountain Lion Without Erasing the Drive
Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.
When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the main menu.
Reinstall Lion/Mountain Lion: Select Reinstall Lion/Mountain Lion and click on the Continue button.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
Interesting thing happened. I got the computer to boot up. I decided to plug my bootable backup HD in the USB port so I could boot from it and do a disk utility repair. When tried to boot toot same problem. Apple scree then grey screen and sets thee. I can get it to do the same thing when I plug my USB fan in.
The Agree Screen !!!!!! What Agree screen? There shouldn't be any Agree Screen when booting a properly working system.
I have a late season 2011 MacBook Pro that is giving me some booting problems. Also have a problem at times if it is on a while will break up the screen display into two or three parts or screen turn a dark blue. Maybe the problems are all related. Any way the boot problem is this and this don't happen all the time. I will turn it on it will get past the Apple screen and then agree screen just stays ther and doesn't boot any further.
Any help or ideas appreciated.
Mail (at) mikealrhughes (dot) com.
Your GPU (HD 6750m or 6770m) is defective. That's a well known problem with the 2011 models.