This is likely a duplicate of issue #845.

Try downgrading your Fabric API version if you've upgraded it recently, it may be too new.

(Also, make sure you aren't using Quilt. OptiFabric only works with Fabric.)

Unfortunately, we can't help without more information.
Could you upload the latest.log (just after the load failure), and what's the version of optifine?

Is working fine for me.