I have a rooted Sony Xperia Z3+.
So I was mucking around in a root terminal (on my phone, not via adb), and I simply typed stop.
The phone seemed to instantly turn off, but it won't turn on again! What did I do? Sorry if this seems like a dumb question, but I cannot even find any infromation on the stop command by googling.
From @AndrewT 's comment, I gathered this:
Referring this answer on StackOverflow
What does stop do?
stop entered in a root terminal basically stops the Android process called zygote. This in turn stops all processes registered in the context of zygote, that is, all services (including system services) registered to zygote. This includes the SurfaceFlingerprocess responsible for rendering the Android UI.
So, nothing displays on your screen once you enter this command, but the Linux kernel below Android is still going, as you can check if you have a UART connection to the device, or have set it up to specially broadcast output to ADB even if zygote has stopped.
A power-on isn't possible after stop as technically the device isn't powered-off.
What is zygote?
zygote is the hosting process for any Android app. That is, it holds the context of the app and the app itself, and on versions of Android below 5.0, hosts the Dalvik VM instance for the app. It is a system-level process, started on boot.
So how can you recover your device?
Try a hard reboot. That is, pull out your battery, leave the device alone for 15 or so minutes, and then put it back in and boot it up again.
If your device lacks a removable battery, the workaround is to hold down the power button until the device vibrates or shows the bootloader splashscreen. This usually takes about anywhere from 10 seconds to 1 minute.
If you can get the bootloader splashscreen, you are (relatively) safe, you aren't hardbricked. Further steps can be added if Android fails to boot past that step. If that happens, and you have a custom recovery and a NANDroid backup, just restore that.
If you can't get the bootloader splashscreen, check if your device charges when connected to a mains source. If it does, you're still relatively safe and a restore is possible (but out of scope of this answer, as it is very device-specific).
If it doesn't even charge, a recovery may still be possible but you are effectively hardbricked. Maybe the service center can help you further.
Note that the cause of the soft or hardbrick wasn't the stop command, probably something else. What, can't be said without further details.
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