That message means your computer isn’t detecting a bootable operating system. This could be due to a misconfigured boot order, a failing hard drive, or corrupted boot files. Here’s what you can try:

1. Check Boot Order in BIOS

Your system might be trying to boot from the wrong device.

• Restart your computer and enter BIOS/UEFI (usually by pressing F2, F12, DEL, or ESC during boot).
• Look for the Boot Order or Boot Priority section.
• Ensure that your hard drive (SSD/HDD) is listed as the first boot device.
• Save and exit.

2. Check if the Hard Drive is Recognized

If your hard drive isn’t listed in BIOS:

• Check physical connections (if you're comfortable opening the case).
• If it’s a desktop, try reseating the SATA/power cables.

3. Repair Bootloader (If Windows Is Installed but Not Booting)

If your hard drive is detected but Windows isn’t booting, you’ll need a Windows recovery tool. Since you don’t have an installation CD, you can create a Windows bootable USB:

• Use another PC to download the Windows Media Creation Tool from Microsoft.
• Create a bootable USB (at least 8GB) using Rufus or the Media Creation Tool.
• Boot from the USB and go to Repair Your Computer → Troubleshoot → Startup Repair.

4. Test the Hard Drive for Failure

If the above steps don’t work, your hard drive may be failing.

• In the recovery environment, open Command Prompt and run:

  bash
  CopyEdit
  chkdsk C: /f /r
  

• If you suspect disk failure, try booting into a Linux live USB to access your files and back them up.

Let me know what happens, and I can guide you further.

If your computer was running fine and suddenly rebooted to this message, here are some possible reasons:

1. Unexpected Power Loss or Crash

• A sudden power outage or system crash might have caused a reboot, and the bootloader isn’t finding the OS.
• If the power supply is unstable, it could lead to random shutdowns.

2. Windows Update or Driver Issue

• Some Windows updates (especially BIOS or driver updates) can cause boot failures.
• If an update was in progress and the system restarted, it might have corrupted the bootloader.

3. Hard Drive Failure or Disconnection

• If the hard drive is failing, it might have stopped being detected by the system.
• A loose SATA cable (for desktops) or SSD connection (for laptops) could also cause this.

4. Malware or Corrupt System Files

• Some malware or disk corruption can damage boot files, leading to boot issues after a restart.
• If you’ve recently installed software or noticed strange behavior before the reboot, this could be a factor.

5. Overheating or Hardware Failure

• If your CPU or GPU overheated, the system might have shut down to protect itself.
• Faulty RAM can also cause unexpected crashes and boot issues.

Next Steps

• Check if the hard drive is detected in BIOS.
• Try booting into Safe Mode (if possible).
• Create a Windows recovery USB to attempt startup repair.

Did you notice any strange behavior before the reboot?

Since it's likely hard drive failure (3) or overheating/hardware failure (5), here’s what I recommend:

Step 1: Check BIOS for Hard Drive Detection

1. Restart your computer and enter BIOS (F2, DEL, or ESC during startup).
2. Look for Boot Options or Storage Information.
3. See if your hard drive (HDD/SSD) is listed.
   • If the drive is missing → It may be loose or failing (go to Step 2).
   • If the drive is present → Bootloader might be corrupted (go to Step 3).

Step 2: Check Physical Connections (If Possible)

If it’s a desktop:

1. Turn off the PC and unplug it.
2. Open the case and re-seat the SATA and power cables to the hard drive.
3. Try a different SATA port if possible.

If it’s a laptop:

• Check if the SSD/HDD is properly seated (if accessible).

Step 3: Check for Overheating Issues

1. Does the PC feel very hot? If yes, dust buildup could be causing overheating.
2. Let the system cool for 15-30 minutes, then try booting again.
3. If the PC starts but shuts down again, it might be thermal shutdown due to CPU/GPU overheating.

Step 4: Boot from a USB Drive to Diagnose

If your hard drive is still not booting but detected in BIOS:

1. Create a Windows bootable USB (using another PC).

2. Boot from the USB and enter Command Prompt (Shift + F10).

3. Run:

   bash
   CopyEdit
   chkdsk C: /f /r
   

   • This checks for disk errors and bad sectors.

4. If you suspect hard drive failure, boot into a Linux live USB and try to back up your data.

Step 5: Replace Hardware (If Necessary)

• If hard drive failure is confirmed → You may need to replace it.
• If overheating persists → Clean out dust and replace thermal paste if necessary.

Let me know what you find in BIOS!