my lumia stuck on spinning gears while trying to reset

[Windows Central Question]

Its on w10 TP and when was trying to reset it due to built 10512 down load was stuck on 0%..it restarted and stuck on spinning gears. The line filss up up to 30% and then it shows sad smile then battery low and the Nokia logo the spinning gears again n again n again.can't hard reset bcoz volume down+ power key doesn't work it just take me to the spinning gears again how to fix it if I use recovery tool so how much data is required for full process including down grading to 8.1?