The ctrl key on my Macbook pro has stopped working at once! I have tested "Show keyboard viewer" from the languages, all keys are shown to be working correctly but not the ctrl key.
Could it be any software issue, or I should check the hardware?
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Power on your Mac while holding the option key on your keyboard. If done correctly, and assuming a firmware password is not set, you should see your internal hard drive show up on the display. This is just to verify what you should see.
Now, power on your Mac while holding the option + control keys. Do you see the same screen that you did when you only held option? If so then your physical control key (really the whole keyboard) is faulty. The logical next step would be to replace the keyboard. If not, your Mac booted normally without showing you the screen in the first step, then your control key is working properly. Since option + control is not an actual modifier when booting it is ignored and the process continues normally, this would tell us that both keys are functional and recognized at the hardware level. At this point you could try repairing or reinstalling your operating system.
Hope that helps narrow it down for you.