The sidebar is still there - enable it by clicking Playlists at the top center of the screen. Its behavior in iTunes 12 is a little different from previous releases, and may take a little getting used to. The main difference is that there is no single sidebar covering all media kinds - it varies based on the media kind selected (via the icons at the top left of the screen and the ellipsis ("...") icon). I would suggest taking a few days to get used to the new layout, I find that it works fine for me.
You can, if you wish, roll back to the last version with the old-style sidebar, using these links to the 11.4 installers:
Note, though, that you'll need to recover the last valid iTunes Library.itl file from your Previous iTunes Libraries folder - it will be named "iTunes Library yyyy-mm-dd.itl" where yyyy-mm-dd is the date on which you updated from 11.x to 12.x. With iTunes not running, move your current iTunes Library.itl file (by default in C:\Users\username\Music\iTunes) to another location or rename it to something like XXXiTunes Library.itl. Then copy the most recent iTunes Library yyyy-mm-dd.itl file to the iTunes folder and rename it "iTunes Library.itl" (or just "iTunes Library" if you're not displaying file extensions in Windows). Then install & start the older version of iTunes.
Note, though, that this process reinstates a "snapshot" of your library database as it was when iTunes was upgraded to 12.x from 11.x - any media you've added, playlists defined/modified, etc. since that date will not be there (as in not visible in iTunes - any media files added will still be in your media folders).
Before starting - even though updating or rolling back to a prior iTunes version shouldn't affect your library content, it is always a good idea to create a backup before starting - see turingtest2's notes on Backup your iTunes for Windows library with SyncToy for a suitable/recommended method.
