Sometimes Windows gets corrupt midway during Windows Updates especially if forced shut down and is unable to boot. The first step in such problems is to select Last Good Known Configuration, the second step is Startup Repair:
Last Known Good Configuration
The first thing to try is selecting Last Good Known Configuration:
Power down your computer if not already powered down and wait at least 10 seconds before powering it back up. When you get to the Dell BIOS screen, tap F8 repeatedly.
This will open the Advanced Boot Options Menu
Use the [↑] and [↓] arrow keys to select Last Good Known Configuration and press [Enter]
Hopefully your computer should Startup, if it still doesn’t startup run Startup Repair.
Startup Repair
You will need a Dell or Retail Windows Vista/7 Reinstallation DVD or bootable USB for this step. If you don't have a Dell Windows Vista/7 Reinstallation DVD then follow the instructions here to make a bootable USB:
Power down your computer if not already powered down and wait at least 10 seconds before powering it back up. When you get to the Dell BIOS screen, press F12.
Select boot from CD/DVD or from USB respectively.
Press any key when prompted such as "h" when it says Press any Key to boot from CD/DVD.
You will get a black screen that says "Windows is Starting Files" then one that says "Windows is Loading Files." These should disappear pretty quickly. Next one that says Starting Windows.
You will then be prompted for your language, time and currency format and keyboard settings. Mine are all set to U.K. as shown below, amend for your preference.
Select Repair Your Computer.
The setup will then search for Windows installations 
It will then find an installation
Likely it will be the C: partition. Click the partition and click next.
Select Startup Repair.
The Startup Repair should run and hopefully fix the problem.
If your problem is still unresolved you will need to run the F12 Pre-Boot Diagnostics to see if the issue is caused by hardware failure.
