Q: grey screen with loading bar.how can i fix this watch the video below

Ok what is happening is this.

You have corruption in your MacintoshHD partition, OS X is loading partially and runs a at boot fsck drive check/repair on the MacintoshHD partition and it's failing, why you get a stuck loading bar while trying to boot.

You booted into RecoveryHD parttion and are trying to install OS X, however it can't be installed in RecoveryHD, nor Windows/BooCamp. Those partitions are not for OS X or don't have the right format.

If you can't over install OS X over itself into the MacintoshHD parititon, which is properly formatted for it, it means there is corruption there in that partition the sructure is damaged and needs to be erased.

If the MacintoshHD partition does't appear, it's also a serious issue and the entire drive will have to be erased to recreate the MacintoshHD partition.

Good news, your drive appears to work fine. It just needs data recovery and reformatting.

This is what your going to have to do.

1: Boot into RecoveryHD and use Disk Utiltiy and OS X installed onto a blank powered external drive, you then can hold the option/alt key to boot from it.

2: If you need to grab your recent files out of the MacintoshHD partition you can do that or use Data Rescue to try to recover from the corrupted system. Also you can use WinClone 3 to clone the BootCamp partition to another external drive.

3: Once you have your OS X users files and BootCamp cloned off, your ready to erase the ENTIRE drive (disk0) using the 3x security overwrite to map off the bad areas, then format and install OS X  just like you did on the external drive.

4: Holding the option/alt key down while booting gets you Startup Manager, where you can select what you want to boot from, external drives, Windows or MacintoshHD. System Preferences > Startup disk will make it pernament.

You can't boot from the cloned Bootcamp, but you can reverse it back on and then boot from it like before.

Create a data recovery/undelete external boot drive

Reset your Mac

Make a bootable clone of your boot drive

Were you booted into your Recovery parition? It sounds as if you tried to erase the drive while mounted.

Clinton

It seems as if your hard drive is damaged. Since you're able to boot into your Recovery parition, but NOT able to reformat the internal drive and are only offered your Recovery partition to restore Lion, that would be my guess.

Looks as if it's time to go shopping for a new internal hard drive... you'll still be able to restore from your backups.

Clinton

Then your drive has failed and will need to be replaced.