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

When i press play it loads then goes straight to a this error
Error: Could not find or load main class net.minecraft.client.main.Main
Java HotSpot(TM) Client VM warning: Using incremental CMS is deprecated and will likely be removed in a future release

Please help.