Contact the oven manufacturer. They might have a good idea.

Warm up the oven just slightly to the point where you can peel it off, or hot enough for it to melt onto the floor of the oven. Put a sheet or two of foil on the oven floor under the rack to catch the melted plastic. This comes from personal experience. :/

I've done something similar before. I would use a culinary torch and an old butter knife. Take it all outside and go to work. You can apply the torch directly to the plastic and allow it to flame and drip off, or heat the butter knife and use it to scrape it off. Just be careful. And keep some water nearby.

The last time we had an issue like that, we were lucky in that the plastic didn't adhere to the racks.

It just kind of flowed over it like petroleum-based taffy. So we broke it apart with pliers and popped it out.

My first (lazy) thought was to see if you can simply purchase a replacement rack.

This happened to us when my husband put a cutting board in the oven ("to dry" he says) and didn't tell me about it when I started to make dinner. For us it popped off after it had cooled and hardened a bit, but our racks have a rubbery (dunno what it is) coating that helped I think.

Heat may be best but might fracture off easily if cold. See what happens if you put in the freezer!

If the plastic doesn't come off, replacement oven racks cost about $20, and can be ordered online for your make and model of oven. Fumes from burning plastic can be toxic, so the investment in a new rack might be worth the money. Instead of sending the old one to landfill, use it to organize your pot lids or CDs or jewlery or Christmas ornaments!

This happened to me and it came off pretty easily once it had completely cooled.

It will smell awful, but you can usually burn it off in a "self-cleaning" oven (extremely high oven temperature). Alternatively, you can do the same thing on an extremely hot outdoor bar-b-q grill.

If your racks have a shiny chrome finish it may be damaged by "burning off" the problem.

Another vote for get a new rack if it doesnt chip off. Plastic gives off very nasty fumes to incluide cyanide, halogens, and other things that like destroying organic matter. If you can chip it off, clean it very very thoroughly to reduce the risk of residual chemicals that could also poison you. Or... Just dont risk it and get a new rack.

To get residual plastic that's melted to the rack\racks, you will need to reheat the oven and use a cold rag to wipe it off as it melts.