my phone wont update because it keeps shutting off, what do i do?

ok so I woke up this morning and my phone wouldn't turn on, it was fully charged when I went to bed and wasn't plugged in during the night. I plugged my phone into my charger it tells me to connect to ITunes so I did through my laptop and it told me that if I update my issues will be resolved because I don't have the newest update (ios8.4) so I tried to do that and it wont let me update or restore my phone and keeps coming back with error 21. I noticed my phone keeps shutting off while it tries to restore or update and that's probably whats causing the error.. so what do I do? im so lost