I had that issue with another plant, not a hydrangea, and I called someone at a local plant nursery for hints. She told me what to do but then offered to help me if I brought the pot to the plant nursery. Since this was not a heavy pot, I took the plant and they got it out. From what I recall, they loosened the soil at the sides with their hands and with some digging instruments (they dug fingers straight down at the edges to loosen the soil and roots located there). Not all the way down, mind you. At a certain point, they turned the fingers inwards and pulled up. Too much resistance that first time so they went 90 degrees from that position and repeated. With their hands digging straight down and then turning the fingers inwards from the sides, they then pulled the plant upwards and it came out. Total time, a few minutes? I expected a more time-consuming and complicated "surgery".
My original plan, I admit, was to break the pot. Hee hee hee.
Note: I have not bought that type of pots since though.