Technical term for the temporary loss of understanding a word
Sounds like what you're describing is "semantic satiation". Wikipedia explains:


  The explanation for the phenomenon was that verbal repetition repeatedly aroused a specific neural pattern in the cortex which corresponds to the meaning of the word. Rapid repetition causes both the peripheral sensorimotor activity and the central neural activation to fire repeatedly, which is known to cause reactive inhibition, hence a reduction in the intensity of the activity with each repetition. 


English.SE also has some good answers related to usage of this term, rather than the neurological/psychological mechanics of the phenomenon itself.