I understand this problem very well as i face many errors in reboot. Some it doesn't began, some time it stop in middle and so on. But i fix it by following method:
Restart the computer.
At the Dell logo during restart, tap the F2 key repeatedly until you see Entering Setup in the top right corner of the screen.
Depending on the BIOS, you may need to press a key listed at the bottom of the screen to load default settings.
Press ESC to exit the BIOS, and be sure to select Save and Exit to save your changes.
If you don't understand these steps properly then read their detail explanation in Dell OptiPlex 9010 Manual. This method fully resolve your rebooting issue.