Symptom

For example

1. You can't type lowercase "s" in the terminal, but you can type capital "S" in the terminal.
2. You can type lowercase "s" anywhere else.
3. You can copy lowercase "s" from somewhere else and paste it in the terminal.
4. Disabling all custom plugins couldn't help.

Cause

You may have assigned "s" to a shortcut in the keymap hence the logic is triggered somehow.

Resolution

For the rescue, you can duplicate your current keymap as a backup, then restore the keymap back to default. Then the issue can be fixed.