OK here is what you should do if it is a software or temporary hardware problem (like loss of NTFS integrity or boot sector on drive):
Go to Microsoft's very own website:
Link Removed
Download the Windows 7 ISO file.
Use Rufus to burn it to your USB drive:
Rufus - Create bootable USB drives the easy way
Run Recovery mode from Windows Setup.
You can also access the command line this way, running:
sfc /scannow
chkdsk
If you are using a MBT partition table you can repair the master boot record:
Restore, Fix, Repair Master Boot Record (MBR) in Windows
This requires some careful work that can mess things up even worse.
Run the automatic repair first!