Your wilted plant looks like it may have reached the permanent wilting point. This is the point where the plant is so dry that no amount of water will revive it. You did the right thing pulling it out of the sun and watering but you need to really soak the soil to try and save the plant. Don't worry if after a few days this plant is still wilted. You tried, it died and now you learned something. Plants in pots dry out fast and small pots dry out faster. In hot weather they could need water twice a day.  Tomatoes need large containers like 5 gallon buckets or larger. You can grow small plants in smaller pots but then transplant them into the ground or into larger pots when about a foot tall. There could be another cause of the severe wilting. If the plant was sprayed with an herbicide, or if salt was placed in the soil or even a very cold night these things can all result in what looks like a severely wilted plant.  The most common reason for a wilted plant however is simply lack of water. Keep growing, Marianne Binetti