Hello, I've updated to nougat my bv7000 pro but reflashed using sp flash tool back to marshmallow because it was too laggy and had problem with gsm signal. After flash even if I hard reset it, it still doesnt go pass the screen. I tried flashing back nougat but I get no command after black screen no matter what I do, even again if I reset and wipe cache.
Someone know a fix or something please?

try a clean flash again. If that don't help flash with "download and format all". (this may delete your imei but it's possible to rewrite.)