No Boot after power off

Navjyoty

Dear Sir,
I have a up squared box with ubuntu installed. After power off and on Box is not booting up with ubuntu and entering in Bios Configuration.

After checking I found that under

Boot --> UEFI Hard Disk Drive BBS Priopities --> Boot Option #1 [Disabled].
After selecting OS option rather than disabled it boot up.

I want to know the reason why it happens and what settings i can do to prevent this problem.So In future I will not face such type of problem.

Thanks
Navjyoty