My auntie's cat once went missing for 5 weeks! To this day we never know why, and she never did it again.
She was a little tubby before so when she came back she was really skinny, but in all in good health.
Like the others say it's good that you've checked with neighbours etc as they can get trapped, but as she is still young then she may just be on an adventure!
Don't give up, cats can look after themselves, and as she is young she can hunt, so fingers crossed she will return soon.