Windows 10 laptop suddently needs to boot csm mode

I recently bought a new asus laptop with windows 10 on it. I did a reinstall (of windows 10) and I have been using it after that for about a month or 2.

Suddenly it boots only to the bios. When I was checking the boot options, none was there. Also my date time was reset. So I assumed that it was a hdd failure or a different kind of hard ware failure. But when I tried to add a new boot option I could browse to the disks/partitions. So after some googling I switched off secure boot and enabled csm and it now boots back into windows!

The questions that I now have are:

1. Why do I have to enable csm in order to boot, while I didn't have to before?
2. Since my date time was reset, could it be that therefore secure boot was broken? And how to fix it?

Thanks in advance.

BHD