Because the words function together as a single adjective before a noun, long-forgotten needs to be hyphenated.
Civilization has ended millennia ago? Um… no. Change that to just civilization ended.
After already mentioning that this character is the last human on Earth, you don’t need to say it again in the very next sentence. Change the second one to and that she’s all alone or something like that.
An ellipsis has three dots, not two (even for the last human on Earth).
I am irrationally pleased that the writer of this example capitalized Earth.
Get rid of the comma after see. (Why would anyone put a comma there? *shakes head*)
After a long-forgotten experiment, the last human on Earth awakes from her cryogenic sleep. She soon realizes that civilization ended millennia ago and that she’s all alone, so she commits suicide… only to see that Heaven and Hell are already closed.