We poured drano (Sodium hydroxide) down our shower drain (according to instructions, as we have many times), and it hardened like cement. Why did this happen, and how can we fix it?

I've had this happen once before a LONG time ago... ( 28 years to be exact)

I'm not going to say what I eventually did because it was extremely dangerous ( as in it could kill you dangerous and I knew it at the time as well) and the products I used just aren't as commonly available in the USA and I am sure there were better alternatives.

Have you tried bailing out what water you can, and pouring a pan of boiling water into it?

As far as the why? I think It was too much too fast that caused it. But if that's wrong someone will correct me.