IPhone 11 locked up. Slide to Power Off is showing but won't respond to sliding. Help!

If you followed the instructions carefully and it didn’t work use the Contact Support link a the top of this page to discuss hardware repair options. But to be certain, here are the instructions with commentary:

• Press and QUICKLY release Volume Up [the “quickly" is very important!]
• Press and QUICKLY release Volume Down [the “quickly" is very important!]
• Hold the button on the right side of the phone until an Apple logo appears [you may need to hold for up to a minute]

This force reboots the phone. If it doesn’t work and you have done it right your phone is broken and needs to be repaired.

Yes, press and release as one action.

Follow the instructions in the support link that you were reading when you posted. Here it is again ➡️ If your iPhone, iPad or iPod touch won't turn on or has frozen – Apple Support