Why did resetting CMOS fix my computer

"Resetting the CMOS" is a bit of a misnomer. Okay, make that a huge misnomer. CMOS refers to a particular method of designing logic networks, it does not refer to anything which can be "reset".

What people refer to as CMOS is actually a small SRAM chip on the motherboard which traditionally was constructed using CMOS logic. This SRAM chip is used to hold firmware settings. All of the configuration information such as boot priority, overclocking settings, fan speeds, and the storage controller mode of operation are stored on this chip. The reason for storing these settings in an SRAM chip rather than alongside the firmware itself on an EEPROM is historical, EEPROMs can only be overwritten so many times before they become unreliable. As you will see below, it will also provide a convenient method of disaster recovery.

SRAM, like DRAM, is volatile. When it loses power, the information stored in it is lost. When your PC is powered off, power to the CPU, hard disks, DRAM, GPUs, etc... is cut. However, power to the system's real time clock and that small SRAM chip is maintained either through a small standby voltage supplied by the power supply or through a small battery located on the motherboard. This is what allows your PC to maintain accurate time down to the second even when it's turned off. It's also what prevents you from having to reconfigure everything when your PC is turned back on.

On the topic of reconfiguring everything, the system firmware does a lot of work to setup the PC into a state in which it can load an operating system. However, it need not do absolutely everything every time it is turned on. The first thing that occurs when the PC is turned on is called POST, or Power-On Self Test. The system firmware performs a number of diagnostic routines to make sure that everything is in working order, and to take a look at what's attached to it. This is a fairly time consuming process so it doesn't make sense to do absolutely everything each time the PC is turned on unless it's absolutely critical. Certain parameters need only be checked or calculated once, at which point they are stored in that SRAM chip until there's cause to revise it. If the firmware is even the slightest bit buggy, it may fail to revise the parameters when there's a hardware change or a failure. Since the firmware is just a program like any thing else, it can freeze up before initializing the graphics adapter, fan controller, or anything else.

Cutting power to that SRAM chip forces the contents of the chip to zero out. All of the volatile parameters are lost. This forces the firmware to start over from the beginning and do everything the long way without taking any shortcuts or making any assumptions. On modern PCs the real time cost is negligible, but this wasn't true decades ago when the IBM PC firmware specification first evolved.