You could have knocked the hard drive's connection loose, which would cause this error as the system wouldn't be able to see the drive, or not see it correctly. It could also be just timing and the problem is not related to being opened up. Then I would look at the following...
Causes of this Error...
1. Incorrect boot order.
2. Partition not set as active.
3. Hard disk failure.
NOTE: Some of the solutions below require the use of the Microsoft Windows setup CD/DVD or USB.
1. Check Boot Order
The simplest fix for this error is to make sure your computer’s boot order correctly list your hard disk as the 1st option.
To check the boot order...
a. Restart your computer
b. Access your BIOS menu.
c. Go to the Boot tab.
d. Change the...