Sudden loss of Service Tag - How to rectify

Booted up my PC yesterday and noticed it was making a loud whirring noise as if a cooling fan or hard drive was spinning at high revs. Never had this issue before. Shut down and rebooted and noticed a screen warning that the PC was running in "manufacturing mode". I was prompted to use Alt + F4 (I think) to switch out of manufacturing mode which I have done.

However, now when I boot up the PC, I get a white screen with the statement:-

"Warning - The Service Tag has not been programmed. The system will not function properly without a service tag.

The setup utility will launch the Service Tag installer.

Underneath are three clickable buttons :- Continue    Bios Set up and  Diagnostics.

I am now having to click the "Continue" button every time to get the PC to boot up.

What has gone wrong and how can I fix this to stop this warning screen appearing every time. BTW I do have the Service Tag info for my PC but don't know how to register this again to get my boot up procedure back to normal.

FYI, last week I had a prompt from Dell that several updates were available for my PC and I ran those. One or all of those updates involved updating the BIOS so I'm wondering if that might have caused the issue.

Help please