Both the past simple and the past continuous refer to completed actions in the past.
Most of the time when we are talking about such actions, we use the past simple. This is by far the most common way of talking about the past.
- I lived there for 6 years.
- I only found out a few moments ago.
- I asked her but she didn't know anything.
- The company made 100 people redundant last year.
Only use the past continuous when you want to emphasize the continuity of the action.
- Everybody was talking about it all evening.
- They were really trying hard but couldn't do it.
- I was thinking about you the other day.
- Were you expecting that to happen?
When we use these two forms in the same sentence, we use the past continuous to talk about the "background action" and the past simple to talk about the shorter completed action.
- It was raining hard when we left the building.
- I was reading the report when you rang.
- He was going out to lunch when I saw him.
- The company was doing well when I last visited it.