In general, it's the negative of "был, была, было, были" - the past of "быть" (to be). However, you should note that unlike positive ones in Russian negative sentences like "There was/were" and "Someone had" the past form of "быть" doesn't agree with nouns in gender and number. Examples: В саду была яблоня - There was an apple-tree in the garden. В саду не было яблони - There was no apple-tree in the garden. У меня были деньги - I had money. У меня не было денег - I had no money. As you can notice in the examples, "не было" requires Genitive case of the noun.

We use verb "не было" when the event did not occur. For example: "I ​​have not had time to learn the lesson" (у меня не было времени выучить урок), or "I have not had the opportunity to come early." (у меня не было возможности приехать пораньше).

"не" с глаголами пишется раздельно

Было it is past form, neuter gender of verb Быть. It is like English verb is. We use Не было when не было is agrees with noun or pronoun of neuter gender Дело не было закончено, этого не было. Or in impersonal sentence like- it was not Вчера не было грозы.