DIAGNOSIS: The flashing red light is the vacuum's way of telling you it has a charging or battery communication issue. It's not getting a proper 'go' signal from its power source.
FIX: First, take the battery pack off the vacuum. Give the metal contacts on both the battery and the vacuum a good wipe with a dry cloth to clear any dust. Next, plug the charging dock directly into the wall, not a power strip. Place the battery firmly back on the dock and let it sit for a solid 4 hours. After that, pop it back on the vacuum and try the power button.
IF THAT DIDN'T WORK: If it's still flashing red, the battery itself might have failed or the charger dock is faulty. Try charging a different battery on your dock if you have one, or try your battery on a friend's Shark dock. That'll tell you which piece of the puzzle is the problem.
HEADS UP: These lithium-ion batteries don't like extreme cold. If the vacuum or battery was stored in a garage or car during winter, let everything warm up to room temperature for a few hours before trying to charge it again.