Ugh, that's the worst feeling. Do you have a printout of your key slides or even just a one-page outline? I started keeping a paper copy in my bag after a similar meltdown (seriously, just bullet points on a notecard). It gives you something to glance at to get back on track without staring at a dead screen. I also email the deck to myself AND save it on my phone as a PDF right before I start, so I can pull it up there if I have to.