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I tried to firmware my 750i phone, but didn't worked out so good.
What did you do? Please post logs (not screenshots).
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but I made a GDFS backup, but I can't use it...
Before or after you started to mess with the phone?
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Also, i don't know how to flash...
There are several good tutorials on this forum which explain flashing step by step. They should be read before attempting to flash....
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and if I "killed" my phone with a GDFS that I've found on the internet, can it be restored?
Only if you've got the tools (service cable/software).