Usb jigs may work, but I had this tooo, a long time ago when I flashed a rom...
Put it on the charger, and you"ll feel the battery gets warm, but dont see anything...
It may be that the battery was almost empty and thus the phone shut off...
Mine was gone for 2 weeks, and I nearly throwed it away...
But then I charged it, maybe it could help, and tried gettin into download mode while on the wall charging, and it worked!!!
My phone was alive after 2 weeks of beiing dead, but I was too noobish at that time to know what excactly I did..
But I did had the same when connecting to pc, it says its a modem
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