dual boot – Unable to select the operating system

I had a little problem with my laptop and I had to send it to the company for repair. I had started my dual boot computer with Ubuntu and Windows .AFAIK, my data was in no way tampered with. But after receiving my laptop, I no longer have the option to select the operating system as before. When I checked, this partition is still there. How can I log in to Ubuntu again?

TIA