You need to download the Chrome Recovery Utility app on another working computer with the Chrome browser (this can be a Windows or Mac, doesn't matter as long as it's Chrome). You can find it in the Chrome app store. If you don't have another computer, ask a friend.
Then you'll need a SD card and reader, or a thumbdrive. DO NOT USE SANDISK BRAND. They do not work. (I know, because I tried and had to go buy a different brand of thumbdisk. Don't know why, but Sandisk does not work with Chrome Recovery Utility).
On the working computer, open up the app in Chrome, and follow the on-screen instructions. You'll need your Chromebook's serial number to set up the recovery disk. Your Chromebook should display it as a part of the error message. One the disk is prepared, you'll need to insert the disk into your Chromebook, and let it do it's thing.
I LITERALLY just had this happen to me on Tuesday this week (7.8.16) with my Hisense Chromebook, so I know this works. I've had mine for a year and it's developed a glitchy battery. Think this caused an improper shutdown and a memory issue to occur, resulting in the OS error.