Don't worry! If you previously declined a new student request, you still have the ability to connect with the student.
From the website:
All declined student requests stay within your “Archived” folder. Access your archived messages by clicking the blue drop-down carrot to the right of the "Messages" header, and select "Archived." Click the name of the student to view their initial request within your conversation thread. You can reply to them as you would normally. If you wish to unarchive the conversation, simply click on the "Unarchive" link in the top right corner of the thread.
From the iOS app:
Access your archived messages by clicking the “Archived” tab at the top right of the screen. Select the student's message, and reply by typing your message in the empty field at the bottom, then click the icon to send.
From the Android app:
Tap the three-bar menu icon in the top left corner of the screen and select "Messages." Tap the Archived icon in the top right corner. Select the name of the student to view their previous request. Respond as you normally would.