The first half hour, a battery discharged that far won't be charging, the charger is in what's called "pre-charge" mode. As B. Diddy said, leave it plugged in for
at least a few hours (preferably turned off) and see if it accepts a charge. If not, or if it discharges fast after it's fully charged, contact
Pixel Phone Help. (Or if there's a
uBreakiFix near you, bring it into them.) (Batteries aren't covered under the warranty.)
And for longest battery life, keep the battery between 40% and 95%. If you can't charge it when it gets to 40%, turn it off. (Install
GSam Battery Monitor and set the low limit to 40%, so you don;t have to keep watching it.)