is it one of those squatty ones with the long hose?
It's all very easy.
First check for a blocked hose. Not switched on, disconnect the hose from the dyson (it's only clipped on). Then turn on. If it sucks in as it should, blockage is defo in the hose some where. If not, disasssemlbe bits inside (filter, etc) see if anything blocking there. if so clear it.
Right the hose. Remove whatever head you have on, and try it on again. If it's OK,blockage is in the head, if not, plockage is in the hose. Run water from tap thru hose if it is until clear.
If it's the head, unscew at the bottom - need a long necked philips probably. Take bottom part off, and just clear any that needs clearing.
If you have a rotating brush on it, leave assembled - I can't advise on this as I have never had cause to remove it.