To use nvflash you need a set of files that are special for your device. If you have not created them before, nvflash won't help you now.
However there are a lot of things people tried successfully to get into fastboot mode. Search a little in the forums. Some quick examples: hold power and vol-down for more than 10 secs, use the reset hole and vol-down, power off and attach the charger holding vol-down, etc.
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