Heater blows cold air,thermostat was changed, hose are good what could it be

Could be a blocked heater core (the part inside the dash that the hot water runs thru), or the heater blend door motor not operating (the part that allows the air from the heater into the vents), or a broken blend door shaft, or something similar. Most of these would require a workshop to attend to them, although you could try flushing out the heater core, by removing the heater hoses from the pipes they go onto at the firewall and directing water at a reasonable pressure (not high pressure like a steam cleaner) into one of the pipes to see if you get a whole lot of crud come out. This may or may not fix the problem and I recommend you take it to a garage and if cost is an issue just tell them you want it diagnosed, and to advise on a price to repair. Give them an upper limit on what you expect to pay for diagnosis as well.