Dangerous. Open the wall and fix it.

Even covered junction boxes cannot be buried.

Agreed. Boxes must be both be fitted with covers AND be accessible. You should have an electrician come inspect the cob job by the contractor who obviously either is a fraud or just criminally stupid in his lack of electrical knowledge.

If he was that sloppy what else did he do wrong? You need to fix it. I guess no inspection was done fas that would certainly have been caught by Any inspector.

What else? Well ... it sounds like he did some shoddy structural work too according to this post on a different forum.

Here is a link that might be useful: Home Repair Thread

Was this work done by licensed electrician or a handyman that said he could "save you some $$$ by doing the electrical too"?