Recently my laptop being restarted again and again, this occurs in both Windows and Ubuntu (I have installed both of them on my laptop). I did not have this problem before when I was using just Windows. I think Ubuntu has changed something in my BIOS but when I restart the BIOS to default factory, the issue is still there.
Can anyone help me finding the issue out please? Is it the hardware problem? What should I do now? (unfortunately there is no laptop repair shop in my area)
It sounds like you did install Ubuntu. Try to boot off of a LiveCD. if the problem persists then you know it is a hardware issue. Does it happen at the exact same time each boot or is it very random?
Random reboots can be because of several things. Physically speaking, it could be bad memory, dust shorting connections, bad power supply, bad battery, loose wire and/or overheating.