"I applied to a college via Early Action/Early Decision. I was unfortunately not accepted, so may I apply a second time as a Regular Decision applicant?"
Whether you can or cannot apply again through Regular Decision depends on the exact wording of the decision that you received (either by email or snail mail).
DENIED: If a candidate is denied outright or rejected in this Early Decision/Early Action stage, then you are not able to submit an application as Regular Decision.
If a college or university believes that you are a borderline candidate, but they are not yet willing (or able) to commit to you during the Early Action/Early Decision process, they will DEFER you. There are a select few schools though, that will not ever defer Early Action/Decision applicants as they will only admit or deny them with the early round.
DEFERRED: If you are deferred, the college will then reevaluate your application, resume, and credentials with the Regular Decision pool. As a deferred candidate, you do not have to reapply and the college or university automatically considers you along with the Regular Decision candidates.
After a deferral, absolutely stay in touch with your admission representative - send any updates or highlights via email, correspond to let them know that you are still eager to potentially attend (if, that is, you still are). And, if you do happen to be denied, know that this is not the end of the world. Instead, take a positive mindset, and refocus your sights on schools that you love and provide you with the best opportunities.
Questions on admissions decisions? On the college process? Do not hesitate to reach out by emailing firstname.lastname@example.org!