iphone 6 drained battery now wont turn on

Hi the battery drained down overnight which I haven't let it do in a while. Now this morning when plugged into laptop its going through a cycle of opening with the apple icon then a double beep then the whirling icon in centre then it switches off. Then it switches on without me touching it and the apple icon appears and so on.

I am letting it run through this cycle over and over again in a hope that it breaks out and works.

I have tried 3 different leads so its not a lead problem.

Can anybody help or is it a trip to the apple shop?

regards

Richard