First exhaust all the options in If your Mac doesn't turn on - Apple Support, which has additional instructions in its Step 4, but stop short of erasing the Mac when you can proceed no farther.
For your information If your Mac doesn't turn on - Apple Support was recently updated, removing some outdated but potentially helpful suggestions in its earlier iteration. At your option, review those steps here: If your Mac doesn't turn on (archived)
Next: before you completely give up on that Mac, determine if you can use Target Disk Mode: Transfer files between two Mac computers using target disk mode - Apple Support.
If you exhaust all those options consider the possibility the Mac's startup disk (flash memory) has suffered a catastrophic failure. Flash memory can fail in that manner, in which case it might as well not even exist. Before reaching that ultimate conclusion, consider having its flash memory module removed so that you can possibly install it in an external device, and possibly (again) using another Mac to retrieve its contents. If and when you reach that conclusion please write back for suggestions.