Boot into DVD Recovery Options Command Line to confirm that Windows 7 or it's 100mb System Reserved partition (if you have it) are marked Active Mark Partition Active
then run Startup Repair - Run up to 3 Separate Times
to see if you can start Windows 7. If not work through these other steps for Troubleshooting Windows 7 Failure to Start
Where did you get Windows 7? Did you burn the DVD yourself? Confirm the ISO then burn another using ImgBurn at 4x speed. If that fails write the ISO to flash stick using Universal USB Installer
If you decide to reinstall and it continues to fail, access the Command Prompt from installer at boot to run Diskpart Clean Command
. Unplug all other HD's and peripherals to try install.