GoKawiilExplainer: effects of a dead CMOS battery on desktop PCs
A failing CMOS battery, typically a CR2032 or CR2025 coin cell, loses its ability to keep a motherboard's clock and BIOS/UEFI settings intact when the system is unplugged. Symptoms include the date resetting to a default like January 1, 2000, boot-time warnings, and BIOS settings such as boot order reverting to factory defaults. Replacing the battery is cheap and simple, and using sleep mode instead of rebooting can temporarily preserve settings on a dead battery.