How can I access my files on Galaxy J1 Ace? (restore "My Files" icon)
Did you try a third-party app? Some phones appears to have no file manager/explorer at all. 
To achieve this just install a third-party file explorer and you're done or if you think your phone had it before, you'll need to factory reset it then the icon will be back.

I'd recommend you to download ES Explorer but there are many other similar apps in the Google Play store.