During monica's appearance on a popular quiz show, the host asked her to name the capital of russia, and monica immediately blanked out. she knew that she knew the answer, but she just couldn't recall it. this illustrates:

The answer is blocking. This can be known as mental block. The blocking effect is the relationship between two spurs, a conditioned stimulus (CS) and an unconditioned stimulus (US) is diminished if, for the period of the conditioning process, the CS is obtainable together with a second CS that has previously been related with the unconditioned stimulus.