2001 hyundai elantra heater only blows cold air,it overheats within a block of driving then car wont start for a couple hours what is the problem

Your engine is overheating? A blocked heater matrix alone wouldn't cause overheating.

There's either a collapsed/perished radiator hose, a radiator blockage or a cylinder head/head gasket problem. A head problem can cause overheating.

Start your engine and remove the cap from the coolant expansion bottle - or radiator cap. Let the engine tick over until the thermostat begins to open. If you're seeing lots of bubbles appear in the coolant this is a sign of a cylinder head gasket failure - exhaust gases are finding their way into the cooling system via the defective gasket and pressurising it.

Also check your oil dipstick. Any greyish/creamy sludge on it suggests that oil is finding its way -via a defective head/gasket into the coolant.

Are you able to do a dry/wet compression test? This will identify any head/gasket problem

Blocked cooling system most likely a stuck thermostat. The coolant is not circulation therefore not getting to heater core. Replace thermostat and it should be fixed