2007 Kia Spectra- "needs engine"

[ShellyMay]

My little KIA is sitting at a mechanic shop right now while I decide it's fate.

Just shy of 140,000 miles with never a hiccup before, and less than 4 weeks after the most recent oil change, my KIA seized. Random Mechanic Guy doesn't know why since he said everything looked fine when he was tinkering around with it. When I brought it in for an oil change I also paid for a full diagnostic and I asked him point blank what I needed to do to get another 100,000 out of the car. (Every car I've owned in the past has broken down slowly approaching 100,000 miles and having a car still running great well past that was making me nervous).
He told me that with regular maintenance he didn't see why the car couldn't go at least that.

Then one day just a few weeks later, stopped at an intersection- the car made a clicking noise that concerned me and moments later -still waiting at the red light- the engine just died. My car hasn't started since.

Now I'm being told I need to replace the engine at an estimated cost of $3400.

Is this something anyone else has experienced?
Is the car worth saving?
Any other suggestions on what kind of options I might have to fix my car?
I'm not very car smart- I just take my car in and pay for whatever the mechanics tell me needs to be done as far as maintenance.
I wanted to do a tear down to figure what exactly happened, but the mechanic (who is not affiliated with KIA in any way) says it would be too expensive and the cheapest way would be just to replace the engine with a rebuilt one.

I just bought the 2014 KIA Soul as my household's second car so I can't afford to pick up another car payment, but I can't come up with that large amount of cash for a USED-REBUILT engine either. /Rant

I miss my car.