The box you checked says "Can someone else claim you as a dependent?" It doesn't say did someone else claim you as a dependent. If you met the requirements to be claimed as a dependent but whoever could claim you chose not to, they still could have claimed you.
If you just made a mistake checking the box, then you can prepare, print and mail an amended return to uncheck the box. Do not uncheck it before 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
An amended return takes 16 weeks to process in normal times.
I can't specifically answer your question about whether the IRS will send a stimulus check after you amend but it would seem that they should.
