If your Samsung Galaxy S25 Ultra is completely black and won't respond, the first thing to try is a force restart. This is different from a normal restart and can clear temporary software glitches that cause a freeze.
Press and hold the Volume Down button and the Side button (what you'd normally use to lock the screen) at the same time. Keep holding them for about 10 to 15 seconds. You should feel a vibration and see the Samsung logo appear as the phone reboots.
Check Your Charger and Cable
A completely dead battery is the most common reason a phone won't turn on. If your S25 Ultra was left uncharged for a long time, it might need a few minutes on the charger before it shows any signs of life. Plug it in and wait at least 10 minutes before trying the power button again.
Since the S25 Ultra supports 45W Super Fast Charging 2.0, using the right gear matters. I'd recommend using Samsung's official 45W EP-T4510 charger and a high-quality USB-C cable. A worn-out cable or a low-power charger from an older device might not provide enough juice to wake the phone from a deep discharge.
Also, try wireless charging. The phone supports Qi2 at up to 15W. Place it on a compatible wireless charger for 20-30 minutes. If the charging coil lights up or you feel warmth, you know power is getting through, which points to a software issue rather than a dead battery.
Boot Into Safe Mode
If the phone turns on but gets stuck on the Samsung logo or in a boot loop, a recently installed app could be the culprit. Booting into Safe Mode temporarily disables all third-party apps, letting you see if one of them is causing the crash.
First, force restart the phone as described earlier. The moment you see the Samsung logo appear on the screen, release the Side button but immediately press and hold the Volume Down button. Keep holding it until the phone finishes booting. You'll see "Safe mode" in the bottom-left corner of the screen.
If the phone starts up fine in Safe Mode, you've confirmed a bad app is the problem. You can then go to Settings > Apps and start uninstalling apps you installed just before the problem began, restarting normally after each one to find the offender.
Clear the System Cache
Corrupted system cache files from an update or an app can prevent a normal boot. Clearing this cache is a safe step that won't delete your personal data like photos or messages. You'll need to access the Android Recovery Menu.
With the phone off, connect it to your computer with a USB-C cable. Then, press and hold the Volume Up button and the Side button together. When the Samsung logo appears, let go of both buttons. You'll see a blue screen with "Installing system update" briefly, then the Android Recovery menu.
Use the Volume Down button to highlight "Wipe cache partition." Press the Side button to select it. Confirm by selecting "Yes" with the Volume buttons and pressing the Side button again. Once it's done, select "Reboot system now."
Consider a Factory Reset
If all else fails, a factory reset is the final software step. This will erase everything on your phone, apps, accounts, photos, files, and return it to its original out-of-the-box state. You should only do this if you have a recent backup or have accepted the loss of your data.
To perform a reset from the recovery menu, follow the steps above to get into the Android Recovery screen. This time, use the volume keys to navigate to "Factory data reset" or "Wipe data/factory reset." Select it with the Side button. You'll have to confirm your choice again on the next screen. The process takes a few minutes, after which you can select "Reboot system now."
For a phone that won't turn on at all, this recovery menu method is your only option. If the phone is responsive enough to get to the settings, you can also go to Settings > General management > Reset > Factory data reset.
Look for Physical Issues
While less common on a new device like the S25 Ultra, physical problems can happen. Inspect the USB-C port for any packed-in lint, dust, or debris that could prevent the charging cable from making a solid connection. You can gently clean it with a dry toothpick or a can of compressed air.
If the phone has been exposed to moisture, even just high humidity, it could cause a short. The S25 Ultra has an IP68 rating, but that's for fresh water, not necessarily other liquids. Look for a tripped moisture detection alert in the port, which would appear as a warning on screen if the phone had any power.
In my experience, if the force restart does nothing and the phone shows absolutely no sign of life, no vibration, no flash of the screen, no charging indicator, even after 30 minutes on a known-good charger, the issue is likely hardware-related. This could be a deeply discharged and damaged battery, a faulty power button, or a motherboard problem.
