A caution on the fastest fix people reach for: disabling DNSSEC at your DNS host does resolve it immediately, and it is a real decision rather than a shrug.
Unsigning a zone that was signed is fine as long as the fingerprint record at the registrar is removed first. Remove the signing while the parent still advertises a key and you have broken it harder than before, and it will stay broken until the parent record is gone and caches expire.
So: parent record first, then unsign. In that order, and with a wait between them.