It has happened to us all. You go out one day and you cannot even manage to get through a basic greeting at the supermarket. What to do? Break down and cry? Give up?
The truth is that humans (and most everything in nature, in fact) are cyclical. Our bodies operate on circadian rhythms, which means the fundamental chemical balance in our brains changes throughout the course of the day. In the field of psychiatry it is accepted that mentally ill patients will have good days and bad days. If you have ever had a family member with Alzheimers' disease you know that the human brain does not always function the same from day to day or even moment to moment.
In neuroscience we have learned that the process of making a decision or recalling a bit of information is, in some ways, a matter of probability. Action potentials, the stuff that thoughts are made of, travel down neurons in the form of an electrical charge. Each neuron is connected to around seven thousand others. Sometimes it amazes me that we are able to have a complete thought at all!
When you get right down to it, it is quite unreasonable for us to expect ourselves to perform at our peak all the time. As interesting as the science may be, though, is of little consolation when you're staring blankly at the supermarket attendant. So here's what I do when I have a "bad speaking day:"
Slow down, smile and apologize that your brain is not working well at the moment (everybody can relate to this and it removes pressure). Knowing how to break the tension in a time like this is your best asset. If you are uncomfortable the other person will sense this and try to spare you. If you dive back in with a grin, though, they will respect you for trying (no matter how badly you speak).
If the bad day won't stop, do something to build up your confidence. Usually a whole bad day stems from a single bad incident. The human brain is very good at letting emotions snowball, effectively turning a proverbial mole-hill into a mountain. If you think about it I bet you'll recall that your "bad speaking days" always start with getting flustered or called out on a simple mistake. The good news is that this confidence factor works in reverse, too. Talking with someone you're familiar with for a minute or two can build your mental fortitude back up.
If you can't seem to get over the feeling, accept it and take it as a sign to spend some time by yourself studying or thinking. Your brain needs time to digest new information, after all. One thing I have learned from my experimentations is that the majority of progress is not linear - it happens in spurts. So don't worry! Your confidence and skill will come back. In fact, it never left.