One of the simplest checks you can carry out before escalating a connection issue is checking that your cables are all plugged in.
If there's no power, check the socket first. You'd be surprised how often a flicked switch is the cause of your outage!
Make sure the cables are plugged in to the back of the ONT and/or the router and they are switched on.
Check that the ethernet cable is plugged into both devices and that it has no visible damage and has no kinks in it.