If my I-601A application is denied, will I be able to file again?

Yes, especially where the foreign national’s circumstances have changed or where they have been the victim of non-lawyers who are not authorized to practice law. But USCIS will expect an applicant who is applying a second time to provide additional evidence of Extreme Hardship or changed circumstances with the second application, above and beyond what was provided in the denied application. So it is best to submit a strong application the first time you apply, as it may be more difficult to get an approval the second time.
 While it is not required to have your application prepared by an immigration lawyer, it is recommended, as an experienced immigration lawyer knows what USCIS is looking for when deciding whether Extreme Hardship exists and will be able to help you put together a stronger application.