17-5767 Operating System Loader has no signature

Hi,

Help! I am at the end of my tether here! 

My m/c is barely 2 months old. I haven't used it much as I am still working on my old laptop. It went into a longish update a day ago. Today, when the battery ran out and instead of showing me an alert, it just shut down. When I reconnected it to the charger, on its own, it ran some tests and said there were no problems detected. Then it rebooted and went into a screen with the message - "Operating System Loader has no signature. Incompatible with SecureBoot. All bootable devices failed. Secure Boot verification."

And the only other option on this screen is to shut down.

I looked at other threads on the forum and have tried the following - 

1. disabled secure boot & rebooted. 

2. let the battery run out and recharge again

3. Reset to default BIOS settings and add a new manual line for BIOS sequence

I do not understand the technical stuff with hardware/software and so haven't tried the more complicated solutions given in the forums.

I am really out of options here (except contacting Dell Support which is my last resort). I would really appreciate it if someone could help me out here. Thanks.

Poonam