DIAGNOSIS The speaker is likely in a deep-discharge state from a depleted battery that's been sitting, or there's a hard reset needed.
FIX Plug it into a high-quality USB-C charger (not a low-power computer port) for at least 30 minutes, then press and hold the power button for 10+ seconds.
IF THAT DIDN'T WORK Try a forced reboot: hold the Volume Up + Play/Pause buttons together for 10 seconds while the speaker is plugged in and charging.
HEADS UP Letting the battery hit 0% and sit for weeks can permanently damage capacity. Use the PartyBoost indicator light to check charge level—it's more reliable than the power button alone.