I was pleased to get two invites today but obviously couldn't accept both. So I looked at both and decided which I would decline, wrote a polite message explaining why I couldn't take on the project and sent it.
Then I went to accept the second project only to find that I had apparently declined it instead of the project I wanted to decline, which was still open. Is there some way I can undo this? Especially because I've now sent a client a completely irrelevant message about why I declined something else.
I'm really sorry to hear about what happened, but I'm afraid is not something you will be able to undo. As an alternative, you could check your declined invites by going to Find Work > Proposals, click on "Archived" and look for the job under "Archived interviews". If you click on the job post and it is not private, you should be able to submit a proposal to the client.