If your Tiaguard is labeled to be mixed in water, then use that mixture as instructions say, and give the medicated water mixture to her several times a day orally if she isn't drinking on her own, until she starts drinking on her own. I will attach a link to an article that explains how to safely administer medications. For more accurate advice please post a picture of the medication you are giving and the instruction label so we can see it clearly. Tiaguard is often used to treat mycoplasma gallisepticum symptoms. That particular illness is chronic and the birds are carriers, so symptoms can recur periodically, usually during times of stress (which can be just about anything for a chicken). So if the medication helps with symptoms (it won't be a cure), then you may need to retreat periodically. MG is also very contagious, so any birds you bring in will be exposed to it. And no birds should leave your property (don't give away or sell them, or sell eggs to hatch). It can also be spread on clothing, equipment and shoes. Birds can be tested for it, to confirm or rule it out. MG does not affect humans.
For your hen that was having trouble laying, putting where you are located generally in the world in your profile, can help with answering some issues. How old is this hen? Did she molt this fall/winter? If a hen stops laying for a period, due to age, season (shorter daylight hours), or molt, then they can have glitches when they start up laying again. Just like with brand new layers, sometimes it can take a bit of time for their bodies to get back in the swing of it. Hopefully it will resolve, but definitely keep an eye on her. If you do have MG in your flock, that can also affect laying and egg quality.
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/