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Good morning, I have tried to upgrade my windows 8 to 10, after I changed the UEFI mode to boot from external devices it stopped working. Now I can't even get back to Uefi settings to change them to default, as laptop says "No operating system found". Can you please help?

Hi there, Recently my Vaio Laptop ran into a problem, and after many unsuccessful attempts, I made a big mistake - I entered BIOS via shift+restart keys in windows 10. I changed the boot option from UEFI to Legacy. Afterwards, my pc stopped with error "Operating system not found". I neither have access to BIOS settings nor to the windows. I recovered my C Drive (windows Drive) using my Acronis True image boot disk (a back-up software) ; although it successfully replaced my windows drive, I still have the same error "Operating system not found". I tried to install a new window 10 using Bootable flash, but didn't works. I had a windows 7 installation disk and tried to install that one, but when it comes to select drives to install, it mentioned that mine is GPT and it is not possible to install windows on GPT (Probably because of changing in bios from UEFI to legacy) shift+restart was the only way works for me to get to the BIOS, other familiar ways mentioned in this article " F1, F2, F10, Delete, Esc, as well as key combinations like Ctrl + Alt + Esc or Ctrl + Alt + Delete" does not work for me. "Assist key" does not work for my VAIO as I removed it from my HDD several years ago. I have a full Backup of my laptop from the purchasing day so am able to bring back assist functions. And maybe afterward my F1/F2/F3 functions well and transfers me to BIOS settings. Anyway, from that time now I changed some partition sizes and I have a purchased a software that worked with the code generated by the current size and info of my HDD. So If I use that scratch copy of my system, my codes and access to that products fails. Therefore, I am looking for some simple way to solve it. I remember from old days that BIOS settings back to default by removing motherboard battery. But I am not a technician and do not know if it is a good idea or not. I am wondering if someone has a solution the problem. Thanks for your time.