First, breathe. Look over the heads of the audience and smile (this releases endorphins and helps you calm down, plus it makes you look calm), but focus your attention on your mind's eye. In you mind's eye, remember when you were practicing the talk. You have about the time it takes to take a breath to reground yourself. If you don't quickly remember what you were saying, try to remember the big picture of your talk. Choose something relevant, make a quick mental outline of what you remember you want to say, and keep talking based on your new outline. If you have notes, you can try to catch up to them the next time you breath. Repeat until you find or remember your place. You can increase the amount of time you have to find your place by slowly taking a sip of water if you have some handy.
If you have power point slides, you can use the slide to help you remember what you need to discuss.
To make recovery easier, practice losing and finding your place while preparing for the talk. Stop and look out the window, or have someone randomly interrupt you. Practice looking composed while your mind races through your talk. If you practice finding your place, your brain will pick up the threads more easily under pressure.
