Translation:There was a computer.
컴퓨터가 있었어요. can also mean
"I had a computer" depending on the context.
If used in another context,
There was a computer. = "컴퓨터가 있었어요. " = 거기에/여기에 컴퓨터가 있었어요
People normally say 거기에/여기에 컴퓨터가 있었어요 rather than 컴퓨터가 있었어요
If a native speaker read "컴퓨터가 있었어요. " alone without any context given, 9 out of 10 will translate as "I had a computer"
있다 doesnt work with 를/을. Both "there was a computer" and "i had a computer" would use 이/가 on "computer". To be more specific, if you want to say "i had a computer" and include "i", the sentence would be: "저는 컴퓨터가 있었어요" If you want to say "i had a computer" and use 를/을, you could use 가지고 있다 and say: "저는 컴퓨터를 가지고 있었어요." notice that this option might be a little less common in past form, but im not entirely sure about it. In present form it is definitely common.
Basically, descriptive verbs (예쁘다, 춥다, 크다, etc) cant act on an object (same with english, if you use an adjective it refers to the subject, but cant refer to an object) therefore you cant use 를/을 with them, and would normally use 이/가 or 는/은. Hope this helps☺