How can I get it back
Depends what "it" is.
Chase it down, run screaming after it, "But I looooove you" and maybe it will come back. Sorry... to what are you referring? (If it's a computer thingy, I usually just run a system restore back to before I lost it and POOF! it's back)
on your knees and beg
If it's meant to be it'll come to you;))
You must accept that now is all you have, its all you may as well have had and its all you ever will have. Nothing is real but this very moment.
you can't it's lost forever
The same way you got it first time!