"We'll take you up to the hospital wing. Madam Pomfrey never asks too many questions...."
--Harry Potter, speaking to Hermione Granger after her Polyjuice Potion mishap (CS12)
Located on the first floor, and later one or two floors up, the hospital wing is where Hogwarts residents go to have their medical needs met, anything from basic first aid after rough Quidditch matches to treatment for serious medical conditions like petrification from a Basilisk. The hospital wing is the domain of the school matron (U.S.: nurse), Madam Pomfrey.
Within the hospital wing are lots and lots of beds with white sheets, privacy screens, and bedpans (Ron once had to scrub them for detention) (PA9). The entrance has double doors (OP32). Madam Pomfrey’s office is part of the hospital wing complex.
In the 1992-1993 school year, Harry leaves the hospital wing, intending to go up to the seventh floor and the Gryffindor common room. He gets to the floor above the hospital wing and he is at Moaning Myrtle's bathroom, which is on the second floor. Later in that chapter it states that they go up the marble staircase, which goes from the ground floor to the first floor, to get to the hospital wing. This places the hospital wing on the first floor (CS13).
By the 1995-1996 school year, the Hospital Wing seems to have moved to the second or third floor, since when Harry runs from the ground floor to the hospital wing he goes up the marble staircase and then up "more flights of stairs":
... Harry ran up the marble staircase, hurtled along the corridors so fast the portraits he passed muttered reproaches, up more flights of stairs, and finally burst like a hurricane through the double doors of the hospital wing, causing Madam Pomfrey - who had been spooning some bright blue liquid into Montague's open mouth - to shriek in alarm (OP32).
It is entirely plausible that the Hospital Wing magically moved to a different floor, but it is also possible that this is simply an inconsistency on the part of the author. Rowling did state that in a magical castle, rooms move around sometimes. It's also possible that with a castle as magically convoluted as Hogwarts, you needs to go up and down staircases just to get to a location which is technically on the same floor you're on.