Thanks for writing. That can be a bit tricky because regular garden soil doesn't usually work out very well in containers and you don't want to distrub the roots too badly when you transplant something. I'd recommend investing in a new hydrangea, personally.
If that won't work then carefully dig out the hydrangea from the ground and wash much of the soil from the rootball. Then plant it up in moist potting mix in your pot on the patio and keep it in the shade for several weeks while it recovers. Water it enough to keep the potting mix evenly moist, but not wet, and expect the plant to look sulky and lose a lot of leaves at first.
----Justin, Senior Garden Editor, BHG.com