In this particular case you have several possible situations. The operating system installation is corrupted preventing the NUC to find boot records, the Adata mSATA SSD drive failed and no longer detected by the NUC, or the mSATA slot in the NUC is faulty.
It is important to make sure that the SSD is being detected properly in the NUC BIOS. If it is detected then most likely your operating system installation failed. If you don’t see the SSD in the BIOS then it is necessary to reset it. For that purpose you need to open the NUC and re-seat the Solid State Drive. If that doesn’t help, test the drive in another computer to make sure it is healthy.
As an additional troubleshooting step, please update the NUC BIOS to the latest version (0040).
Download the latest BIOS file using the following link: https://downloadcenter.intel.com/downloads/eula/24991/BIOS-Update-WYLPT10H-86A-?httpDown=https%3A%2F%2Fdownloadmirror.intel.com%2F24991%2Feng%2FWY0040.BIO
Update the BIOS using the following instructions: http://www.intel.com/support/motherboards/desktop/sb/CS-034522.htm