If you have NOT e-filed yet it is easy to fix---go to My Info, click your name and go to the screen that asks if you can be claimed as someone else's dependent. Say you can be claimed.
If you already e-filed then your problem is bigger. You will have to wait until the return has been fully processed and amend the return. Meanwhile.....the person who CAN claim you is not going to be able to e-file and will have to mai their return this year. Waiting for you to amend will not help them----once you put your SSN into the system it is in there for the year. So....if someone who has not yet filed is expecting to claim you as a dependent---warn them now that they cannot e-file this year.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**