There's been a lot of chatter on Community about a variety of cards and actions that have stopped working. One thing you might try that seems to work for people is to delete and rebuild. If it's practical to do so, you might want to give that a try.
Re: Notification stopped working: Notification failed to send. Please try again
Thanks for flagging this - we have a development team currently pushing through a fix for this which will extend the 100 character limit and we'll be releasing this as soon as possible. We'll also work on some improved UI for the builder and hope to get this change out shortly after.