coolant no heat

My 93 cherokee ran low on coolant and the low coolant warning came on the screen while my friend was using my car. When I got the car back he said it was low on coolant so he filled the radiator with water. Now the temp gauge in the jeep won't raise above the first line from the left side and only blows out cold air. I was told to get a radiator flush and it will fix the problem I have. It's been like that for about 3 months and it's now 20 degreez outside and I need to be able to warm the inside up. help. thanx (The heater DOES blows air but it's cold air.)

the therastat is probably stuck in the open position, the stat has to keep the coolant in the engine until it is warm,about 200, if your temp gauge was in the normal position i would suspect the heater core, but with your temp gauge low i would replace the therastat, cheap fix.