My iPhone 4s turned off and now won't come back on?

Earlier my iPhone 4s turned off while I was using it - hours after updating my IOS to 7.4.

I thought it was just out of charge, because I hadn't taken any notice of how much it had. When I found my charger I plugged it in and it came on. It came up with the Apple sign for a bit and I unlocked it when it turned on. I was about to click onto iMessage but then went off. I kept it plugged in and it went off again, and again. It just keeps doing this but the Apple logo will come up and then it will just go blank and try turn on again. It doesn't do anything when the charger isn't in and it won't come on when I connect it to my laptop.

It is second hand but it's not damaged in any way and it has worked perfectly up until now. My mum thinks it's the IOS 7.4 update I did this morning but I don't know.

I've tried resetting it but nothing comes up because it won't turn on properly.

I managed to look at my iMessages and I found that my friend thinks I'm ignoring her, I tried to reply and it went off and won't even stay on long enough for me to click it! Please help!:'(

Added (1). Videodude747,

It won't connect to my computer: L

Did you hold down the menu button and top but until it restarted?

I sugest this option if you can manage to put it in DFU mode

A full restore.

This will totally wipe out the whole ios and all your data. And then it will download the ios and install it from scratch

Connect to pc,
Open itunes
Switch iphone off
Hold the Power button for 3 seconds
Hold the Home button without releasing the Power button for 10 seconds
Release the Power Button but keep holding the Home button
Keep holding the Home button until you are alerted by iTunes saying that it has detected a device in Recovery Mode

If you are not alerted by itunes, try it again.

If you are sure its not working, then see the 2 option down below. If it worked then read on…

Now its in DFU mode, you need to restore the phone. Remember this will delete all your data, however, if you made a back in the past few days or weeks. You can put that info back on when the device is on again. You can also put the icloud data back on.

Anyway, on itunes press restore, you will then get a few pop ups, telling you about the firmware it will put on your phone (in your case ios 7.0.4) it will always put the latest software on if you restore

Now it will download the firmware.

Once downloaded it will install

When its installed, goto your phone and check if it works, go through the setup. Get your data back from icloud, or get it back from a back you made a diffrent day.

If your phone didn't get into DFU, it is certain that its a battery problem.

Next option would be to visit an apple shop. Book an appoinment in a Apple shop near you. These guys will give you the best answer and they may repair it for a resionable price or sometimes they will replace it with a new/refrebished phone for free.

They will also give you the reason why it may have happened. Tell them about your ios 7.0.4 update

I updated my iPhone 5 to ios 7.0.4 this morning and its still working fine right now