First thing to check is the coffee basket that sits in the portafilter. Remove it and hold it up to a light; you should be able to see through all the little holes in the bottom; if most of them are clogged this is most likely your problem. If the basket is clean and water comes through the portafliter just fine when there's no coffee in it, try a coarser grind as well as little to no tamping. If coffee still won't flow then your pump may be failing and will need to be replaced.