Whether the port's peripheral clock is enabled in RCC. MCUs gate the clock to every peripheral off by default to save power, and a GPIO port with no clock won't respond, you can write MODER/ODR and see no effect, and reads may return zero. The fix is to set the GPIOx-enable bit in the appropriate RCC AHB/APB enable register before configuring the port. After that, check the obvious config: is MODER set to output (01) for an output, or input (00) with the right PUPDR for an input; for an LED, is it actually wired the way you think (active-high vs active-low, current-limiting resistor present); and if it's meant to be a peripheral pin, is MODER set to alternate-function with the correct AFR selection rather than plain output. But the first and most common culprit, especially for newcomers, is the missing RCC clock enable.
Embedded Systems Fundamentals · Interview question
You configured a GPIO pin correctly but it does nothing. What's the first thing to check?
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From the lesson
GPIO: Digital I/O
Reading buttons and driving LEDs: configuring a pin's direction, push-pull vs open-drain outputs, pull-up/down inputs, and why a floating input reads noise.