In the past when this happened, I was able to google the answer with no problem, but this time nothing's coming up.
Letter keys are normally bound to
self-insert-command, so you can simply evaluate this:
(global-set-key (kbd "x") #'self-insert-command)
Of course, you'll have to replace that
x with whatever key you want to rebind. You can eval this by yanking it into a scratch buffer, putting point after it and typing C-x C-e or you can enter it in the M-: prompt.