Wilting is a sign of transplant shock; a severe change in environment, or damage to the roots, may cause issues like this, too. In your case, I'm thinking it's just not hardened off to your windowsill. Whatever the cause the solution is often (not always) the same. Extra leafy plants can be especially problematic, since they require more water than plants with fewer leaves (and they can't get as much water due to the transplant shock, or whatever the issue is). Leaves that have endured drought can sometimes be permanently altered in their function.
I would suggest the following:
- Trim off some leaves to eat (ideally the biggest, oldest, wilting ones).
- Mist your plant (it helps to get some water to the leaves)
- Give it some potassium sulfate or wood ash (potassium helps plants to absorb water)
- Don't give it any nitrogen at this point (that can make issues like transplant shock worse)
- If you want to be extra cautious, reduce light levels for a while (but reducing the number of leaves should reduce the light the plant is getting; new leaves that grow should be adapted)
Bamboo's suggestion of just keeping it out of bright sunlight should be sufficient, though! But I'd personally give it some potassium, too, at least.
If the plants are too immature, removing some leaves may not help.
The trouble with supermarket herbs is, they're surrounded by a plastic bag which supports the foliage - when you remove that outer wrap, they often flop over completely. Usually, with basil and others, you can water and they will stand up again, but my coriander (cilantro as you call it there) looks very similar to yours, bought about 4 or 5 days ago, and hasn't stood up. It's not as bad as yours, but then mine's not in sunlight, its on a windowsill that provides good light but no sun, so I suggest you move it out of the sun, because it won't be helping.
These plants (usually more than one, several seedlings initially, over-fertilized and forced to grow on to provide quick foliage for sale, unlike ones sold in garden centres which are proper plants) seem to require frequent watering, so make sure you do that, but don't leave it sitting in water after 30 minutes, it won't help and may cause fungal problems. Use the foliage in cooking and when you do, cut it off near the base to force new growth - as it regrows, it should then be upright and you may even be able to let it have some sun through that window. And I don't recommend giving any fertilizer at all - it will have been overdosed before sale anyway.