Diagnosis
That's the low battery indicator. The most common cause is the charging contacts on the handle and charger base aren't making a solid connection, so it never gets a full charge.
How to Fix It
First, give it a good, long charge. 1) Unplug the charger base from the wall. 2) Wipe the two metal charging pins on the base AND the two little metal contacts on the bottom of the brush handle with a dry cotton swab. Any film or toothpaste gunk breaks the connection. 3) Plug the base back in and seat the handle firmly. You should see a solid red light on the base. Leave it for a full 24 hours. The blinking red on the handle should stop and it'll turn on.
If That Didn't Work
Try a different outlet and make sure you're using the original power adapter that came in the box. If the base light doesn't come on at all with the handle removed, the power adapter or base might be dead.
Heads Up
Don't store the handle on the charger 24/7. Waterpik recommends charging it only when the battery is low to prolong battery life. Once it's fully charged (solid blue light), take it off the base.