There is nothing more frustrating than a crack in a piece of acrylic sheet. Because this materials is “notch sensitive”. By the way, so is glass! That’s why you can scribe and break glass (and Plexiglas®) along the scribe. These materials have no grain and therefore cracks can travel as they please. However, cracks can be stopped with a simple procedure. Using a very small drill bit, (1/16″ or so) drill a hole through the material at the end of the crack. That’s it. If it is a long crack, you might be able to inject some IPS WeldOn® #3 (Methylene Chloride) into the crack to partially seal it. If the plastic is on a sign, and therefore most likely opaque, you should glue a 1″ strip of 3/16″ to the back side to reinforce the cracked area.