My heater in my 2000 Lincoln LS only blows cold

Have your cooling system inspected. No heat means the water is not circulating to the heater core which is located in the firewall. You may have a plugged system. Low coolant levels will also cause this problem. Make sure it's topped off before you spend money to have a mechanic look at it. Make sure your overflow tank is topped off.

I've done all that and it still blows cold air. It was blowing hot till a week ago when it overheated. I installed an electric fan for the overheating problem 6 days ago but I can't figure out the heater issue

What was the original cause of the overheating condition?

I'm thinking it may be a stuck thermostat.. Without the thermostat opening all of the way (or not at all), the engine temperature will rise not allowing the system to open and allow flow through the radiator and or heater core. Is your engine still running hotter than normal? Is your system building pressure after full engine temperature is achieved? Aside from that, you may have an issue with your thermostatic control inside the vehicle. Vacuum driven damper, bad switch on the dash type issue? It wouldn't have anything to do with the over heating condition other than pure coincidence.