If the float is a rectangular white plastic thing with the pickup pipe going through it weighed down with a thick steel ring over the end of the pipe......then it's upside down in the beer, with the pickup pipe facing upwards into the headspace rather than down into the beer.
Sterilise some marigolds (outside only
) crack open the cap, put on the gloves, turn the float over. If it resists then pull the pipe off the drum tap and re-position it orientated so the float naturally rests the right way up, ie pipe down into the beer. Put the cap back on, pressure up with Co2, wait a few minutes, then gently open the cap until the gas escapes. That purges the air out of the headspace. Re-seal, and re-pressure. Normal service resumes.
If it's the other sort of float..it shouldn't happen unless the pipe has seperated from the float, in which case, repeat the method above except that you just refit the pipe to the float.
Good luck
I'm not sure what has happened but it just lets the gas out when I turn on the tap. have i missed somthing obvious? The keg was a hand me down from my dad and there are no intructions. please help me as i want to try my brew!! sorry if this makes me a numpty 
Sanitising the gloves is kinder to your hands, and a bit more sanitary, because once you open your keg with your sanitary hand...you could pick up a bug off the cap and straightaway become 'unsanitary'. With the gloves, you open the cap, then put the gloves on avoiding touching the business end. No infection possible.
result beer now flows properly, no spluttering. Must remember to add that to the list of pre barreling checks!!
Well, in the end, the problem was solved. That's the main thing. Don't forget to purge the air out before settling down for a drinky