Translation:It was so cold that I could not sleep.
In a simple declarative sentence, the verb takes second place. Ich konnte nicht schlafen / I could not sleep. The inclusion of the subordinating conjunction, dass, now makes this a subordinate clause. In a subordinate clause, the verb which would normally take second place, konnte, moves to the end. http://german.about.com/library/weekly/aa010910a.htm
"Konnte" is the past-tense, while "könnte" is the subjunctive. While they both translate to "could," alternate translations exist that distinguish them: "konnte" can be read as "was able to," while "könnte" can be read as "would be able to."
"Es war so kalt, dass ich nicht schlafen konnte." = it was so cold that I was not able to/could not sleep
"Wie könnte ich schlafen, wenn es so kalt wäre?" = how would I be able to/could I sleep if it were that cold?