F12 is a one time boot menu and it doesn't save the last boot information  You need to change it in BIOS  F2 > Boot  Once you have set it to "windows boot manager" go to Exit and choose "Save changes and RESET"    if changes in BIOS also not saving up after you shutdown the machine than only you need a new CMOS battery.  
Suddenly my XPS that was working fine, doesn't boot.  The problem arose after I was alerted that the time/date was wrong and had to reset the time. Now, every time I boot, I get the error "No Bootable Drive"  I can use F12 to get inot the "one time boot menu."  Legacy Boot -- hard drive is selected. If I go down to UEFI Boot: Window's Boot Manager and hit ENTER, the computer boots up normally. But the next restart is back to square one.  How can I fix this?
No bootable drive, must use F12 and change to UEFI Boot every time