Many people have reported that windows 8.1 tablets like Toshiba encore 2 and Dell venue 8 pro,
Somtimes do not wake up from sleep or do not turn on after leaving them for charge during the night.
The only solution that solved my problem was updating BIOS.
Toshiba Encore WT8-B32CNM :
http://support.toshiba.com/support/modelHome?freeText=1200008324
Dell Venue 8 Pro:
http://www.dell.com/support/home/us/en/19/product-support/product/dell-venue-8-pro/drivers
If your tablet doesn't turn on try this method to turn it on:
1- Plug in your tablet to AC power for 5 min
2- Hold Power button for 15 Sec
3- release power button
4- Press and hold it again until it turn on