when plug my phone into socket it says 'connect to itunes' when plug it into m computer all tht happens little apple icon flashes on and off what should

when i plugged my phone into a socket it came up 'plug into itunes', when i took it out of the charger it turned of straight away (even though it had full charge). i plugged it into my computer and since then it just flashed the little apple symbol on and off without actually turning on. what should i do? please help!