First step is to find if Checkbox is ticked/checked. to do that you can use below line of code.
driver.find_element_by_id('<check_box_id>').is_selected()
Next step is to idenfity if text filed is disabled. to check that you can use below code.
driver.findElement(by).isEnabled()
So your code should look like this.
if(driver.find_element_by_id('<check_box_id>').is_selected()){
if(!driver.findElement(by).isEnabled()) System.out.println("Checkbox is checked, textbox is disabled.");
}else
{
if(driver.findElement("Textbox locator here").isEnabled()) {
driver.findElement("Textbox locator here").sendKeys("I can enter keys");
System.out.println("Checkbox is checked, textbox is disabled.");
}
}