It's been a year since I have my computer in Windows 10 with an Ubuntu dual boot which worked perfectly until then. However, I suddenly no longer had the choice of operating system at startup and when I tried to see my ubuntu in the boot list with F2 at startup I only found Windows.
https://image.noelshack.com/fichiers/2020/46/3/1605115050-img-20201111-180425.jpg
My computer is an Acer aspire 5 (A515-51-56VN).
Thanks you for your help, Vincent