Power is out

Check the fuse box/circuit breaker

Blower belt is broken

Replace the belt

Blower motor is broken

Call a qualified professional

Filter is dirty

Replace the filter

Registers are dirty

Clean the registers

Air leaks in the ducts

Seal the ducts with duct tape

Heat exchanger is dirty

Call a qualified professional

Filter is dirty

Replace the filter

Heat exchanger is damaged

Call a qualified professional

Ducts are dirty

Have a professional clean them

Pulley is loose

Tighten the setscrews

Blower mounts are worn

Put in new blower mounts

Worn belt Replace the belt

Belt tension is too tight Loosen the belt tension