I worked hard in a museum and I was lied about unjustly and I want to have my say. What can I do?

There's a problem. The museum is a business, businesses are run as the owners see fit, and, quite frankly, "having your say" is pretty meaningless at this point. If they are a bunch of liars, distance yourself completely from them and do your own thing without them. If you fight them, it just adds fuel to their fire and they will just lie more. The truth comes out. You do NOT have to be the one to force it. Walk away, proud and true to yourself.

Still there? Make a grievance. Left? Move on.