This is the 9345th most frequent Japanese word.
眠れ
"眠れ" means "Sleep!" or "Go to sleep!" (imperative form).
Here, "眠れなかった" refers to the inability to fall asleep, expressing the negative past form of the verb.
昨晩は眠れなかったので、今日はとても眠いです。
I couldn't sleep last night, so I'm very sleepy today.
In this context, "眠れる" is used in its potential form, indicating the ability to sleep.
この薬を飲めばすぐに眠れるでしょう。
If you take this medicine, you should be able to sleep quickly.
"眠れました" here is the polite past form indicating the speaker successfully fell asleep.