Christmas Ale Woes

[Tiger Ed]

Brewed 22 ltrs of spiced Old English Ale for Christmas and it tastes wonderful but its as flat as water.
After fermentation in which it hit its FG I transferred it to a Young's plastic keg (with a new cap and seals) and left it at 20c for 6 weeks then dropped to 12c for another 2 weeks. On Christmas eve I drew off a pint and "Nooooooooo" it was FLAT. I fired in a co2 bulb and it shot 1/2 out of the blow off valve so it must be under pressure. What do I do now its too good to throw away.

[DEMPSEY]

if it is under pressure then chill it and it will absorb the co2 gas