My Iphone tried to update and now wont turn on

Hello,

I am hoping someone could help me. I have an iphone 5s and since the new I.O.S 8.3 update has come out it hasnt been able to update. It kept saying it was unable to due to a problem. Today however my phone has decided to automatically do the update. After being off for a minute it told me to plug my phone into Itunes. After doing so Itunes said it was doing the update but keeps coming up with the same error message that was previsously showing on my phone.

Now after half a day of this happening my phone will not update and i cannot turn it on as it just shows the pictures to plug it into Itunes.

Unsure what to do now.

Please Help!