My cat had 5 kittens two weeks ago and this past weekend she ran away with 1 of the kittens. Two days later she returned but did not bring her other kitten. We believe she didn't leave the neighborhood cause our neighbors saw her walking around each night in the neighborhood but did not want to be approached by people.

What can I do to try to find the lost kitten? I'm scared it will die if it hasn't already :(

You could ask the neighbors where they saw her going and go to those places and listen for the kitten.
Did she leave the other 4 for 2 days without coming back to feed them?
About the only thing I can think of is walking and listening. If there is a hungry kitten around it will no doubt be crying.
You will have to look high and low and in every nook and cranny.
Unfortunately you may have to prepare yourself for not finding her.
If you cannot find her and the litter is outside, I would consider bringing mom and kittens indoors so that she can't relocate more making it impossible to find them.
Good luck I really hope you find the kitten and she is well.

We've talked to the neighbors and they couldn't see where she was sneaking off to. And we haven't heard a peep yet from the kitten in the neighborhood. We are constantly walking and driving around listening for faint cries but no luck so far.

When she ran away, she left the 4 kittens alone but we fed them with kitten milk from the pet store and droppers. We also placed teddy bears in their bed for warmth. Now that she's returned and we placed her back in the house, we are scared the other kitten is going to be hungry and might not survive through tonight if we don't find her. We are praying that will not happen... thanks for the support.

Keeping my fingers crossed that you find her.