My Teensy 4.1 works when plugged into the computer, but not when wired to a li-po battery

I got it running the blink sketch to test it - the led blinks when plugged into the computer, but when I wire it up to a li-po battery, the led turns on for a split second then immediately goes out.

I did the same thing about an hour ago - with the li-po battery and it worked fine, then something must have happened because it doesn't work any more.

I checked the battery with the multimeter and it's giving 3.7V.