American citizen cannot be denied entry to the USA period. However there is a minor possiblity that CBP can give you a hard time, search your electronic devices etc for proof of any activity against US. There have been multiple cases of US citizens subjected to such scrutiny (google it). But it’s relatively rare, I’m sure Iranian Americans visit their home country often without issues.
As of now, most flights are grounded. Even if you are able to find a flight back, expect to be asked to quarantine for 14 days especially since Iran is a hot spot. At this point, the state department has a travel advisory that says “Do not travel”. This is not just limited to Iran but to all countries including Canada. This is a fluid and evolving situation. No one knows how it will be 2 months from now...could be better or worse.