94 Cav has smoke pouring thru vents

Hello,
for the past week or 2, I've noticed a slight "burning" smell coming from the vents when I start up my 94 Cav with 120K. It always passed, so I didn't worry. Today, thick smoke started pouring thru the vents! It was scary! No smoke was visible in the engine compartment. The coolant level was empty too. I had the core replaced a year or so ago. Small "critters" sometimes are seen in and around the engine compartment. This summer, I noticed the car ran hot a lot, but I thought maybe thermostat and since I don't drive it a lot, I didn't do anything about it. I am grateful for any suggestions and help. Thank you for your time!