Tablet is just with power and volume buttons and any of known common combination works, but he dont want to go further from ANDROID letters loop. PC can see it (two disks). but not in ADB (so, no chance to force recovery reboot from PC).
Mainly, chinese manufacturers (for me) for unknown reason apply one (or booth) of two hidden options (some other is not excluded, but 99.99% exist):
1) Performing a simple pin-hole reset (so, look for very tiny hole, much more tiny that mic hole), usually placed near the power button.
2) Simple extended power press upgrade. Procedure is more than simple. Just press and hold power button till appear dialog with system upgrade (you should not to do anything else, tablet will be recovered itself).
Hopefully this will help for peoples in boot loop problems.