IBM Fellow John Cohn shares a tale of two talks: one that sucked, and another that rocked. He says:

I don't know.. all that I know is that really empty feeling of being half way through a talk .. all eyes on you.. and you just know that you're sucking big time.. You can't gracefully just stop.. though
perhaps that would be better than continuing.. maybe the best thing to
do in a circumstance like that is to reach for the fire alarm and jump
out a window.

It's reassuring to know that even seasoned tech celebrities (he's got an awesome TV show, even!) have panic moments like that. =)

I know that feeling. I've run into that a couple of times, and it's never any fun. One time, I was just a few minutes into a talk for high school students when I realized that the presentation I prepared was likely to bore me, not to mention the tough crowd. So I threw out my slides, turned off the projector, gave people a quick idea of what I knew about, and had a great conversation instead.

When in doubt, listen and improvise. (Which I'm sure John Cohn has done more times and more effectively than I ever have!)

If you find yourself unavoidably sucking at a presentation, don't be so hard on yourself afterwards. You propose a topic, the organizer accepts it, and people usually have a choice of whether or not to attend – and certainly, whether or not to pay attention. If one of these points fail – maybe you or the organizers misread the audience, maybe people just aren't having a good day – that doesn't make you any less awesome. Keep trying.