game won't load

Whenever I try to open minecraft, the launcher shows a progress bar that says "Downloading 1.8.4.jar...", gets about 90% done, then shows me an error summary that says

Error: Could not find or load main class net.minecraft.client.main.Main
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release