I have a problem with power

It's working now, eh? What did you do that got it sort of working?

If it's constantly rebooting we refer to that as a boot loop. iPhones have a standard reaction to unrecoverable hardware or software errors; they reboot the phone. As it turns out, the phones scan their sensors every three minutes, so if you've got an issue with one of the sensors, that's enough time for the phone to boot, run for a little while, then after three minutes it's time to scan the sensors. Oops, there's an issue; think I'll reboot. Rinse and repeat ad infinitum.

Fortunately, iOS logs those reboots and the reason for them. One of iFixit's contributors has written an excellent Wiki article on those reboots, or "kernel panics" as they're technically termed. Along with explaining the causes behind them, there's also information about how to find the logs that will show you why the phone rebooted.

iPhone Kernel Panics - iFixit

So my suggestion is to read the article, go retrieve your panic log or logs and either screenshot or cut'n'paste the first page or so and add it to your question. We can take a look and see if we can tell you what you need to do or replace to get your phone working again.

Adding images to an existing question - iFixit Repair Guide