Only goes to boot and app screen and has 2 choices SanDisk or boot mgr

Make an USB bootable and reinstall the OS.

@stnational your Chromebook may show a Windows Boot Manager but it is to not intended for dual booting. Since it does not boot from the drive that contains your OS (which is your Sandisk DA4064) it is possible that this is an OS error. First make sure that your drive still how in the BIOS. Next try to boot from a bootable USB drive by pressing F12 during startup. If that works and if your drive still shows up in the BIOS, you know it is your OS.

If you want to you can try to recover your OS. Here are the instructions for that:

1. Using the Google Chrome browser on another computer, download and install the Chromebook Recovery Utility by selecting Add to Chrome in the Chrome Web Store.
2. Launch the program. You’re asked to provide the Chromebook model number, which you can enter manually or choose from a list.
3. You may be asked to grant permission to make changes to your USB flash drive or SD card. Simply click Yes.
4. Plug the USB flash drive or SD card into the Chromebook, and then follow the on-screen prompts to download Chrome OS to your external drive or card.
5. Once the download is complete, remove the USB flash drive or SD card.
6. With the Chromebook powered on, hold Esc+Refresh on the keyboard, then press the Power button to reboot in recovery mode.
7. At the Chrome OS is missing or damaged screen, insert the SD card or USB drive containing Chrome OS. You should then go through the operating system’s installation process.

from here