I bought my Umi Super 3 weeks ago.
Last night the battery was at 15% but I did not put it to charge before I went to sleep. This morning I thought that the battery needed recharging, but even after recharging for 2 hours I cannot turn it on.
had this problem once on a nexus 7 2012, the solution for me was to jump start the battery, here is a link to the best umi super tear down video i know of:-
and here a link to a video of jump starting a nexus 7 battery, which will show you the principles of what to do, and of course you will need a spare USB cable:-