Translation:Si tu congèles les restes, ils ne sont pas perdus.
English example should use the future tense 'will be' for 'seront'
Why is "ces" not okay here in place of "ils"?
"ces" is a determiner meaning "these" or "those", not a pronoun meaning "they". It can't be used by itself, but must go with a noun, e.g. "these leftovers".