the unions sometimes gets corrupted the only way to fix it is to dupe your union and put it into another place if it does not work its gone
Oh no, your union has been corrupted! This is a common issue, nothing to worry about. There is a Devforum article, here. I have personally experienced this, and this method works perfectly. Please give it a shot.
If you don't know how to insert .RBXMX files (part of the process,) please visit this ScriptingHelpers post!