Boolean Testing Unit (B.T.U.) with Voltage Regulator

Let’s think about this scenario: It is a Sunday, close to midnight, and you haven’t finished with that circuit you need for your Digital course lab for tomorrow morning. Hours later, tired but finally with the final circuit done on your breadboard (i.e. proto board) and… The logic is not right, the LED should be ON now…but it’s not…the true table is right, you “de-Morgan-ized” (I love this word) correctly, you double checked the logic with a simulator… but the logic is still not right. So, time to troubleshoot. You think that you wired incorrectly your logic gates but no, they are right. Could they be damaged? How do I know for sure? What was the true table for the XNOR gate again?”