Good morning 2liptulip & SLOB503.
I hope that by now there has been some kind of resolution to your devices being frozen in USB Download Mode. If not, let's get a fix for you now. When powering the device on or off, were you possibly touching/ holding down another key at the same time? Doing so could cause your phone for go into USB Download Mode. If possible, you can take your device into a local store to be flashed. You can find the closest store to you at the following link: http://bit.ly/3SdsA
We also need to verify there is not physical/ liquid damage, and if under warranty, we may need to send you a replacement. If this has not been resolved, please follow me and send me a Direct Message, and I will be happy to personally follow up with you.
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I had the same problem today.
Just carrying the phone in my pocket turned off.
When I tried to use it, the screen was frozen with the message
"USB Download Mode....Bootblock version..."
I got it back to normal operation by removing the battery and replacing it after several hours
(a minute or less might have been sufficient, but I thought I'd play it safe since I had plenty of time).
This also just happened to me. I thought I turned my phone off. Later tonight I looked at my phone and the screen had the
"USB Download Mode....Bootblock version..." message. I took the battery out and the SD card. The screen went blank. I replaced both the battery and SD card after just a few seconds and turned the phone on. It started normally.