Hi Slinger,
Thank you for reaching Microsoft Community.
The device may be in a hung state. Pressing the Power button for a longer time forces the device to shutdown completely and restart. Performing this is like removing the battery from a laptop or unplugging a desktop PC. It resets the hardware and clears all the cache and memory for the hardware. To avoid the issue to happen again, please make sure the device is up to date and no corrupted software on it by running these steps below:
1: Run Surface Diagnostic Toolkit and check Windows Update
2: Update drivers and firmware
Check what is the current OS Build of your Windows version by going to Settings > System > About.
Go to Download driver and firmware for Surface, choose Surface Pro 3 then click Download on the next page.
Choose the .msi file that matches your current OS Build and click Next.
After downloading, run and install the .msi then Restart.
3: RunSystem File Checker Tool in Windows 10
Hope this helps.