i covered the plant with cling film with a small hole for watering/air. could it be possible that the heat and film suffocated the plant?

Yes - you baked it in a solar oven.

I don't know if it will recover - I would put it in the shade for a day or two and see what happens.

If it survives, get a moisture meter and use it - water the plant enough to keep the soil moist, but not soaking wet all the time.

i covered the plant with cling film with a small hole for watering/air. could it be possible that the heat and film suffocated the plant?

Yes - you baked it in a solar oven.

I don't know if it will recover - I would put it in the shade for a day or two and see what happens.

If it survives, get a moisture meter and use it - water the plant enough to keep the soil moist, but not soaking wet all the time.

Yeah although watermelons like sun they also like air flow and lots of water i mean LOTS. in my experience they like alot of room for their roots to push down as well that's why big raised beds and ground growing is best for them.

Here is a link that might be useful: TheItalian Garden

How long that plastic wrap was on the container and how much sun it was exposed to is the determining factor about whether the plant was cooked or not and if the picture is of the plant afterwards it does not appear to have been.
What is the potting soil the plant is growing in like? How much moisture does that potting soil have? Too wet is as bad as too dry and will present very similar signs and symptoms.