We have exactly the same problem which I suffered almost a month. Infact, I consulted several cellphone repair stalls but unfortunately no one has able to fix it. But last night, after so many retries, I was able to somehow fix it on my own. take note to be quick to tap before "USER... pops up. Follow below.
1. connect it to internet connection
2. go to Google play store
3. search to install "USER SYSTEM FRAMEWORK BY APUS" or APUS Launcher. (((((perhaps the simplest way to return to normal is to install it again

... )))))
4. Restart the unit.
Try it. The apus launcher is still on my system and scared of deleting or unisntalling to avoid the previous problem.