Citrus plants don't like to get completely dry. I'd recommend you lay it on its side on some newspaper, and slip the root ball out of the pot to have a look. Be careful, you don't want to tear or break any roots.
The plant's water needs would increase - maybe double - when it goes from a lush, humid greenhouse to your dry home environment. They probably told you to water "once a week" but that's probably not nearly enough. I'm betting when you see the root ball, it will be a solid dry brick of soil. You need to put it back into the pot, stand the pot in a bucket or the kitchen sink, and soak the whole root ball, right up to the rim of the pot in lukewarm water for an hour or two.
Once you are sure it is well soaked, and have let it drain (let the water out of the sink, or dump the bucket, then leave it sit in there for a little while so the excess water drains from the pot) then you need to lift the plant and see how heavy it is when it is well watered. Now you know how much it weighs when it's just been watered, you need to lift it every day and when it is getting lighter (the plant is using up the water in the soil) water it and keep watering until the root ball is soaked again. Sticking your finger into the soil is also a good practice to see if the soil is drying out.
Putting the whole little tree into the shower once in a while is a very good practice too. It will not only hydrate the leaves nicely, it will also wash off any beginnings of insect infestations that tend to bother indoor tropical plants in the winter.
Going forward, if you can manage to increase the humidity around the plant that will help it to not dry out as fast, and to keep its nice healthy, shiny leaves too. An evaporation tray of pebbles for it to sit on, with water below the level of the tops of the pebbles will help somewhat. Spraying the whole area as often as possible (just keep a spray bottle of water around there and mist it whenever you walk by) is also good.
I'm with Sue, don't put it in the direct sun until the leaves perk up and it is drinking again. Then move it gradually until it is right in the direct light from the window.