This can sometimes be caused by the Pi not being able to auto-detect a screen resolution, and stalls at that stage of the boot-up process.
The steps below show options to resolve this issue, which should be relevant for all models of the Raspberry Pi:
Plug in an HDMI cable - If possible and if you do have an HDMI cable and a monitor on hand, this is an easy way to rule out the headless issue if you're troubleshooting boot problems.
Edit the config file (force hotplug) - Adding the line hdmi_force_hotplug=1 to /boot/config.txt has solved the issue for some users. You can edit the config file by entering sudo nano /boot/config.txt in a terminal window.
Force a screen resolution - Some users have had success with forcing a resolution in the raspi-config tool. You can do this by entering sudo raspi-config in a terminal window then selecting Display Options > Resolution and choosing a low resolution from the list.
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