Why my phone is not turning on..

I have samsung s9 and the same thing happened when i press power and volume down its showing samsung and then it stops restarting at t mobile and ut doesnt black out or it doesn't restart and then when my phone discharged i pressed bixby, power and volume down it took me to the same thing above the one with volume up to os and volume down to restart and had a warning sign that this custom os can affect my phone and the installed applications. When i tried volume up with power it shows me downloading.. and something’s written on top left hand corner. What should i do…now it is not at all showing anything other than t mobile..