When you add an image to your profile, Nyx relies on the given URL to be embedded, or an image that you uploaded. Nyx does not save or download any of these images set, so if both of those are not available anymore, the image won't show.
This is very easily fixable, you can always upload the image in Nyx's DMs, copy the URL and set that as your profile's image, or you can upload it somewhere else that you are sure won't get rid of it.
Note: Websites like personal drives (such as MEGA.nz or Google Drive) will not work, unless you can get a public permanent URL to said image. (awoo.ml / nyx short urls work, but are not supported completely)