How's this for Sod's Law: I take my Galaxy S2 away on holiday with me, knowing it's going on eBay when I return, only for it to break while on holiday!
From what I can tell I think some sand got in the USB port/charger and screwed it up. To cap it all off it recently came out of its warranty period. Doh.
The symptoms of my broken phone were that it would pause charging and say "Voltage too high". It would keep telling me this every few seconds, meaning the phone can't sleep. Nor could I turn off the phone, as it would just reboot itself. It still worked but was unusable in effect.
Anyway, as always, Google is your friend and I quickly found other people with the same issue. The only solution I found that seemed it would work is to replace the USB port, which you can buy on eBay for about a fiver.
So that's what I opted to do. Myself. Sending it off for repair would make it un-economically-viable for then selling afterward.
First thing I did was watch a video of how it's done, which made me realise I didn't have the right tools for the job. I had some tiny screwdrivers but nothing to safely ply it open without causing visible damage. So, again on eBay I searched for Mobile Phone Case Opening Repair Kit and bought what you see below for 4 GBP.
If you are planning on taking a phone apart I can't recommend the above kit enough! Those green plying tools are brilliant!
Before long my S2 looked like this:
And within 15 or 20 fiddly minutes it was back together the new part in place and all working. Total cost of repair - ~10 GBP.
Don't be afraid to try this at home...