This happened to me. So with a little help I stripped it down and cleaned it out. Sucks like new now. But it's a fiddly job.
Full how-to instructions with photos here:
http://www.dyson-online.com/technical_advice.htm
They also sell parts and tools to make the job easier.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vacuum cleaners.