black screen iPhone 4s

looking for help!

my 4s was launch day white 32gb in uk so few months old.

perfect condition and no knocks etc

but as of last night has started black screening mid use (normally in apps)

just fades out but battery almost fully charged

and doesnt comeback on from buttons or charging

have read about the hard reset (home and Power Buttons for 10 secs) and tried that but only seems to work after being off for quite a while.

it was several hours the first time before reset worked.

this doesnt work everytime and has the phone still turns itself off mid use the same way several times since

one thing to note is that when it does come back on it asks for the phone unlock code but not the sim unlock? does this mean the phone isnt actually 'dead' and in fact has power to the phone but not to the screen?

any advice or help is greatly appreciated as would rather not spend hours on tech calls to find out an answer already out there!

also this only started last night so is it a software upgrade caused it or an app - or is it something phsical/electronic within the phone?

thanks in advance for any advice