Suddenly smelling burning plastic is often en electrical issue such as overheating plugs, overheating cords or wires or even an overheating motor.
If you continue to smell burning plastic you should check your cords, plugs, wires and even your breaker box etc and try to find the source and if you can't find the source you may need to contact the fire department to be safe.
Sometimes a washing machine or even a furnace vent, gas range or oven or electric range or oven may emit a burning plastic smell as well.
I once started smelling burning plastic in my home and could not source it and eventually I started to smell it more near my couch.
I had an electrical outlet behind my couch that was actually getting hot and so I moved the couch out and felt the outside of the electrical outlet and it sure enough was getting hot and so I shut the power off and let it cool down.
Turns out the electric outlet had some loose wires on the hot side of the outlet causing it to overheat due to the high electrical resistance.
I also had a breaker get hot before when it was overloading and not cutting off like it should.
Another reason for suddenly smelling burning plastic is someone burning some plastic outside or something else that is plastic is getting hot somewhere close to your house.
And some people also have what is known as phantom smells where they smell things that are not there including burning plastic but you should always check it out to be safe.