Error trying to start PC Aspire TC-885

Mersaultdies

Hi guys

When I switch my pc on it says 'secure boot violation. Invalid signature detected. Check secure boot policy in setup'

When I click 'ok' (the only option), the next screen says 'reboot and select proper boot device or insert boot media in selected device and press a key'.
The PC is Acer Aspire TC-885, roughly one year old and has had only light use.
I have tried a couple of YouTube fixes involving the BIOS menus but nothing has changed. 
Any help would be appreciated.
THANKS

Edited the content to hide personal information. Also, model name was added to the title.

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@Mersaultdies
Have you made recent changes to the PC or it had the same problem from day one?

Mersaultdies

I have made no changes since I last used it. It has normally been fine. There is one possible cause but I'm not sure how it would have contributed specifically.
The last time it was shut down was by the main power button as there was a problem with the monitor when I powered up and I couldn't see the screen. I don't know at that stage whether the pc was already showing the error or was normal until after resolving the monitor issue.

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@Mersaultdies
For the mean time, there are not much you can do. Try to reset the BIOS back to factory default and see if that will solve the problem. Also, check for loose connections (particularly the HDD).