As you are saying, you are not able to access any internal component of your system not even command prompt so you have very few solutions to fix your system without formatting the system drive. First solution is running SFC command to repair the damaged system files and the second one is repair Windows 8.1 system.
To do this, follow the below given Method:
Run SFC command using installation disc of Windows 8.1
Start your PC or Laptop and from the BIOS boot menu, select the DVD drive.
Now insert the bootable disc of Windows 8.1 operating system in the DVD tray and boot your system. You will see a message “Press any key to boot from CD/DVD” press the Enter key.
After the Windows Setup loads you have to select the language and keyboard and press Next.
Avoid clicking on the Install now button and instead of it select “Repair your computer.”
Here you will be asked to select an option. Press the Troubleshoot key.
Various troubleshooting options will appear, from here you have to select the Advanced options.
Again a list will appear with different options. Click or tap Command Prompt.
Now you have to run the SFC command. To do this type the below given command and press the Enter key to run this command.
This command will take few minutes to complete, wait until the completion of this command and then try to boot your system.
If all ok then enjoy otherwise use the bootable disc and reinstall your Windows 8.1 system.
Source: Top 5 Tested Solutions to Fix “Disk Boot Failure, Insert System Disk & Press Enter” Issue