Flat beer advice required.

I have been drinking my beer which is in my pressure barrel and is perfect for my first attemp.
As the beer level is going down i have been adding the gas to keep it pressurised so it dont go flat etc.
I went away for 5 days and my brother decided that he and his mates would have a drink or two.
When i got home i decided to bottle what was left in the pressure barrel and capped all the bottles,not aware of my brothers session at this time.
Basically the next day i opened a bottle and poured it into the glass where there was no head and tasted a little flat and a lack of bubbles.
He then told me what he had done.
My question is from the time i left it all ok to the pouring of the bottled pint which was 6 days is there anything i can do to the beer in the other 15 bottles to get a bit of fizz/life back into them or is it to late.

Thanks and sorry about the long story.

You can try carb tabs from your LHBS. They're little sugar pills that will need a few weeks at room temp to carb the bottles.

In order to keep the carbonation at about 2.5 volumes of CO2 you need to have the pressure at the correct level for the temperature. Say you have the refrigerator at 36F the CO2 pressure needs to be at 10 LBS per sq in. You need a chart to look up the proper pressure for your temperature.

Pressure chart: http://www.zahmnagel.com/pdf/Beer.pdf

im with Kung up there, try Priming tabs, recap, and give them a shake to stir things up... but the again you said they were only a little flat so that might be to much and youd get bottle bombs, and im not even sure if you have yeast enough to carb naturally anyway... did you cold crash multiple times? or possiable stablize your beer with somthing that would kill your yeast? by the sounds of it you had it in a pressure bottle in the fridge??? and it was without equal pressure for the ammount of brew for awhile??? im guessing you had yeast drop out of solution while in your cooled pressure container, if you had any left to begin with... so Huston i dont know, you may have a problem

cheers

Once it had finished it brewing steps i moved the barrel into the shed and over the last 2 weeks it has been around 4 degrees here so the yeast may have fallen out etc like you say.
I will put them in a warm room etc for a week and see what happens.

Other than that i think it may be wasted.

I went to my local shop but was out of stock of the tablets and would not have any for a week at least they said.
I decided to remove all the caps and i then added a small amount of dilouted sugar/water solution to each bottle and then put new caps on the bottles.
I have then given them a good shake and put them in a box which is covered with a nice big towel should they go pop in the night.
The room temperaute at which they are now in is 24 degrees c or 76 degrees f.
How long would i need to leave them there to carbinate before i try one and see if there is any improvement.
If it works all well and good if not then well i tried.

I carried out the brewing procedure spot on and did not take any short cuts.
When the beer had finished brewing and ready for consumption i put it into the shed.
I tried a pint a few days later and it was perfect,i assume at this stage if i had left it for a month say the next pint would of been as good as the first ?

I think i will carry out the same procedure next time as i think i done that part pretty good but i will put the beer into bottles as opposed to a pressure barrel then at least this way they will all be fine for some time and wont happen again.

Am i correct in thinking that and also advise on duration for carbination.

Cheers and bottoms up.