A "reboot" is shutting the computer off (either via normal means or pressing down the power button for five or more seconds) and starting it up again.
If that is what you did, then your iTunes Library should still be there.
If you mean you reinstalled Mac OS X with "reboot", then you need to be more precise what you actually did with what Mac OS X version and what Mac.
Do you have problem finding your music?
Have you looked into Macintosh HD / Users / YOU / Music / iTunes for your music and the iTunes library?
And your thread title suggests you want to know about how to back up your iTunes library and music, of which there are numerous guides around the www.
But your post suggests, that you lost your iTunes library (or even music) and can't get back to it.
So, what is it?