What type of Dell product is it ? What is the exact problem . Does it flash USB logo or what is happening ...why don't you have an official image you can flash in fastboot / download mode ( I mean one you didn't dump yourself but one from Dell )
More info needed * but this it OT because you should abandon fastboot unless you find an unmodified image to flash
Your first step is to find an unlock code from Dell or find and follow the official Dell restore instructions and methods or MINIMUM find a stock ROM for your device .
Open a thread using the full device details and condition requesting assistance.