91 Cadillac sedan deville failed the compression test on 1991 Cadillac DeVille

I replaced it with a new battery, did an engine tune up and oil change and the problems started....before this my car was running perfectly.
I took it to the dealership and I was informed that my engine failed the compression test. They also found out that the previous mechanic who did my engine tune up used different brands of spark plugs and they did not look like new. Now they want me to just kiss my car good bye. I am devasted...Please help with any advice.

Asked by for the 1991 Cadillac DeVille
A compression test shows if the rings on the pistons are good. If they allow to much blow by the car will not run correctly. If the wrong plugs were used then the air/fuel mixtuer may have not been burning correctly. Thats my best educated guess. It will be costly to have the rings replaced and it you may want to get a second opinion. You said it started after I did the tune-up, did someone else do it or do you do it. It sounds to me like you had it done before and you did it yourself this time?
The tuneup, battery replacement and oil change was done by a mechanic who has his own garage in his home....I don't know anything about tune up etc etc and therefore believed him when he charged me $150 for an oil change and engine tune up, little realizing that he cheated me. Now when I try calling him he does not answer his phone and I live quite far from where he is.....I thought it would be better for me to take it to a reliable cadillac dealership and they called me and informed me that my engine failed the compression test as 4 cylinders failed to fire...1,4,7 and 8 and my best bet would be to say good bye to it. I am now trying to look online for a rebuilt engine and and various other shops have sent me quotations saying they have a OEM engine but after the first incident I don't feel like trusting anyone. My car is in excellent condition interior and exterior wise as it was taken care of like a baby and all the functions and every single switch is functional. That's why I am devasted even to think of saying good bye to it.