Apple Watch does not turn back on

The problem is almost certainly a very discharged battery. See If your Apple Watch won't charge or it won't turn on - Apple Support and remember your Series 4 should still be covered by warranty, so if a recharged battery doesn’t solve the problem Contact Support to arrange service.

Charge your watch for at least one-half hour then do a reset again. If that doesn't help, call an Apple store and make an appointment with their Genius Bar.