I don't know why you can't enter your BIOS, but the Secure Boot Issue can be resolved by installing this Update: Update removes the "Windows 8.1 SecureBoot isn't configured correctly" watermark in Windows 8.1 and Windows Server 2012 R2.
If you run "wuapp" and update the Machine with the latest available Updates the above mentioned Update will be included.
Hello Herman_59, we have seen this issue recently when customers are using W8.1. This was already sent to our Engineering team in order to find a solution for it.
You may want to try the following troubleshooting provided by a customer in this thread. https://communities.intel.com/thread/44769
We were able to replicate every step in our systems, at the end after 8 to 10 hours without the CMOS battery, we turned ON the system and we were able to access BIOS and reset default settings.
You may try the suggestion Nebta gave you since it is provided by Microsoft.
Unfortunelately, none of the above mentioned suggestions lead to a solution.
The final and only solution was the the replacement of the complete mother board.