I yesterday I filed my taxes saying I wasn't dependent, and they were rejected because my parents' tax attorney filed me as dependent. So I went in and changed mine without thinking. Just found out today that I shouldn't have even been claimed in the first place and now don't qualify for the stimulus check, and I'm really frustrated because my parents didn't even tell me about it.
Obviously since I just filed I haven't gotten my refund yet, but is there any way to challenge my parents' claim now?
You are not eligible for the stimulus payment if you checked the box on your tax return that you can be claimed by someone else. You must check the box if you meet the requirements to be claimed by someone else even if they don't claim you. See this for whether someone can claim you or not: https://blog.turbotax.intuit.com/tax-deductions-and-credits-2/family/who-can-i-claim-as-a-dependent-...
If you do not meet the requirements to be claimed by someone else, you can prepare, print, and mail an amended return form 1040X to "uncheck" the box. DO NOT uncheck the box until you start the amended return. Here is a link to help you: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
Amended returns must be printed and mailed. They can't be efiled. You must print and mail it yourself. In normal times, it takes the IRS about 16 weeks to process an amended return. There are taking much longer than 16 weeks now due to the slowdown of IRS processing.
An alternative for you is to wait until you file you 2020 tax return and not check the box on that return. The stimulus payment is actually an advance on a credit claimed on your 2020 tax return. You could get it when you file your 2020 return.
Unless your parents also file an amended return taking you off their tax return as a dependent, both you and your parents will receive requests from the IRS later this year or early next year for information supporting the claims for who could claim you. Whoever wins will get to keep the tax benefits that result. Whoever loses will have to pay back benefits they received and could be subject to interest and penalties.