Let me know if this solution works for you.
Other things to try:
1. Make sure to tap the power button if this doesn't work, you may have put the Surface back to sleep! An easy way to check if the surface is awake is if the typecover keys are lit.
2. Also try Ctrl + Alt + Delete
3. If all else fails, you can restart the Surface by pressing and holding the power button for 10 seconds.