Your machine cannot find a good operating system. Your hard drive may have died or you may have a corrupt system folder. Boot your machine from your installation DVD. Go to the second screen pull down menu to Utilities > Disk Utility. Select your hard drive in the left hand column and see if you can repair it. If you see no hard drive it's time to replace the drive. Here's how to do it: MacBook Pro 15" Unibody Mid 2010 Hard Drive Replacement

if it is showing padlock when trying to boot to an external drive it means there is a firmware password - if you know the password enter it and boot to dvd

if you don't know the password you need to change the RAM configuration and reset the PRAM to remove the firmware password altogether. you can find info on this on apple support pages

Change RAM config = basically take a stick of ram out of the machine

Reset PRAM = with ONE RAM stick removed, switch on the machine HOLDING DOWN BUTTONS CMD + ALT + P + R

wait for the machine to reboot a few times. put ram stick back in and boot to external drive normally.

Try this:

1- start the MBP holding down the 2 keys ->command->s

hold the keys until you seen a black screen with white text

2- type: fsck -fy (there's a space between k and -

3- wait, the computer will scan the hard drive searching for errors. Could take time to complete the task (few minutes).

4- when you get the text: <<The volume "hard disk name" appears to be OK>> the HD should be repaired if it's a software issue. If you get the text: <<System file has been modified>> redo #2 until you see only this text: <<The volume "hard disk name" appears to be OK>>

5- once the HD repaired, type: reboot to load the OS.

This is the same procedure as using Disk Utility to repair a system software issue.