Deleted system and no entry sign I just set up my new MacBook Air laptop, however I set it up towards the wrong Apple ID. In order to just start the laptop from scratch again so I could make a new Apple ID, I reset the laptop. 

By doing so I had to delete the system drive, then re download it as macOS Sierra, which took around 10 hours to download, and then it said download failed. I tried again and it didn't work. I then pressed "help" or whatever it was, the Safari icon, but after I closed it I was left on a blank grey screen, nothing I could do or press. I pressed the power button to turn it off, the only option I had, and when I turn it back on, a no entry sign appears.
With the machine off, hold alt-cmd-R as you turn the computer on to launch Internet Recovery. This will download and install a new copy of macOS.