It's great to have you here, @sjohnson2.
When the check hasn’t been cashed, and wasn’t included in your previous reconciliation, you can void it. Let me guide you on how to do it.
However, if the check was included in your previous reconciliations, you’ll need to undo the reconciliation then delete the transaction. Once done, you can then redo the reconciliation process.
As always, I recommend getting the help of an accountant before doing so.
Feel free to leave your comments if you have other questions. I'm always here to help.