samsung s9 plus

As long as you haven’t dropped the phone or it’s not been exposed to liquid damage, the underlying cause can just be due to firmware crashes which can be corrected in your end. To pass through the black screen of death, try the following:
1. charge the phone to ensure that it has enough battery life
2. if you think that your phone has enough power, do this: press and hold the Volume Down button and the Power key for at least 10 minutes and release when the phone boots up completely.
If your phone remains unresponsive and you are still stuck on the black screen of death, try to troubleshoot further. Here’s what you need to do.
1. Press and hold the Volume Down button and never let it go.
2. While holding down the Volume down button, press and hold the Power key.
3. Continue holding both keys for at least 10 second and release the keys when the Samsung logo appears.
If neither of these options works, charge your phone using the original charger. While charging (regardless if the phone is responding or not), simultaneously press and hold the Volume Down button and the Power key for at least 10 seconds until the phone boots up.
Note: If you notice that your phone heats up while doing the last option, immediately disconnect your phone from the charger.
If you have done all these tweaks to no avail, you should schedule a service repair or bring your phone to the nearest Samsung store so a technician will be able to assess your device and repair it when necessary.