This issue normally happens when you install a mod incorrectly, you need to check the folders you have put your mod in or just reinstall it. This can be fixed by reloading your backup minecraft.jar if you have one. To reload it just paste your back up minecraft.jar into your bins and then it should be fixed. Then Try reinstalling the mod and make sure you have the right version, and you have deleted META-INF file. If that doesn't work then try a different program to load the mod instead of mod loader.
