@danix803 I've found these instructions, I don't have a Sero 7, so read them before try it (to see if those steps make sense to you).
Hold power and volume down while its powered off for 10 seconds ...if the hisense sign comes up keep holding it down until you see the screen flash and its off again and release and press it again and repeat it until you see the green android guy and then power + vol up...for 3-5 seconds until menu comes up
Good luck!