My PC screen is completely black after changing the IBOS settings from Legacy.

Hi James

Welcome to Microsoft Community.

According to your description, I understand that after you changed “Legacy” to “Secure Boot” in the BIOS, your computer screen became black and you can't even enter the BIOS at the moment, so I understand how you feel very well!

This may be due to the fact that when you changed the BIOS settings from Legacy to UEFI mode (which is usually required to enable Secure Boot), your operating system (OS) may have been configured to boot in Legacy mode. If the OS is not set up for UEFI, this change may cause boot problems, including the system still booting with a black screen.

Here are some of my suggestions that I hope will help. However, if you still can't get into the BIOS, I suggest you promptly synchronize this situation by sending it to the brand's after-sales or tech support team, who are more familiar with their brand's products and may have had other users report similar problems. Maybe they have already written a fix or a more targeted solution.

Option 1: We try to perform a power cycle (discharge operation) for your computer.

(1) Make sure the computer is completely turned off

(2) Remove all external connections, including: keyboard, mouse, etc. If you have a laptop, you will need to remove the charging adapter. If you have a laptop, you will need to additionally remove the charging adapter.

(3) Press and hold the power on button (power button) for about 30s to a minute or so.

(4) Turn on the computer normally.

Repeat the above operation 4~5 times to ensure complete discharge.

Option 2: Try different keys to enter BIOS.

The F10 key is a common key to enter BIOS, but it may vary depending on the motherboard manufacturer. You can try the following keys.

For example: Del (Delete), F2, F12, Esc.

Click the key continuously immediately after turning on the computer.

We can also check the official motherboard brand name to see if there are other keys to enter BIOS.

Option 3: If you cannot access the BIOS at all, you may need to reset the BIOS by clearing the CMOS.

The jumpers are located on the motherboard and you may need to refer to the motherboard manual for specific details. To remove the CMOS battery, locate the small round battery on the motherboard, remove it for a few minutes, then reinsert it.

Please note: Clearing the CMOS is considered a high-risk operation and may even cause damage to the motherboard or other hardware if not done properly!

Therefore I recommend you to refer to the video or article tutorials provided by the motherboard brand's official website, or seek help from offline technicians.

Option 4: Make sure all internal components (RAM, GPU, cables) are securely connected. Loose hardware can sometimes cause boot problems.

Option 5: Verify that the monitor is working properly using another system or connection type (HDMI, DisplayPort, VGA). If there are multiple outputs (e.g., integrated graphics and a dedicated GPU), make sure the monitor is connected to the correct output.

Option 6: If you suspect that the computer is still trying to deal with some issue (such as an update), it is best to leave the computer on for a while. If there is no change after a reasonable amount of time (about 10-15 minutes), consider shutting down the computer again.

Assuming that the problem persists, I suggest that you also synchronize the issue with the brand's after-sales or technical support team, who are more familiar with their brand's products and may have had other users report similar problems. Perhaps they have already written a fix or a more targeted solution.

I sincerely hope that the above information will help you. Please feel free to contact me (Photos related to the question would be great!).

I look forward to hearing back from you. 

Best Regards

Arthur Sheng | Microsoft Community Support Specialist