Have I been ghosted?

[Triplefivetriplenine]

I went to a job interview last week and the employer said they would contact me today letting me know if I had been successful. They initially said I would hear from them in the morning but I still haven’t heard. How long should I leave it before contacting them first? I was under the impression the interview went really well

I would wait a while. It just needs one of the people involved on the decision process to have something important to come up to push to process back. If you haven't heard by the end of the week I might enquire then.

If it has been a week, it is highly unlikely it has been successful. In my experience, successful candidates are contacted within 24h.
But if thry are waiting on sign-off etc then it would not be unusual for planned deadlines to shift.
I would wait until tomorrow and phone.

I would say it's not unusual to not hear back from someone on the date they give.

Could be down to the fact they've offered it to someone else and they're waiting for them to accept. Or the workload of the person hiring. Or HR processing things their side. There could be all kinds of reasons.

I'm not an expert, and my advice may well be bad 😂 but personally I'd give it a day or two and send an email asking if there's any update.

I say we'll contact you within a week but usually contact the preferred candidate that day. Our most recent job she was waiting on another interview (which was better for her so no worries), she took it, we went to second place and there was nothing that odd about it. If someone has no chance I let them know ASAP.
I suspect they are waiting on their first choice candidate or some internal sign off.

Not necessarily. They may have multiple interviews to do on multiple days. They can't make a decision until all candidates are seen.