Hello, superschwein22. Endless Summer is a reblooming hydrangea. As all mopheads do, it develops flower buds in July-August and these bloom in Spring (provided the stems survive the winter months). If the stems do not survive, new ones grow from the crown and these will develop flower buds that will bloom around June. Still more blooms can be expected in the Fall. Pruning instructions in the ES Website suggest that deadheading is actually good for this plant as deadheading will trigger production of new flower buds and therefore new blooms. I have not timed how long it takes for new blooms to appear on reblooming hydrangeas but say 6 weeks maybe?
I suspect your plant turned brown because it was close enough to the Fall and it decided to go dormant. On year one, you can expect shrubs to do things later or earlier than normal. It is not a problem; the plants are just "confused" because they are forced to bloom at unnatural times by the plant wholesalers and they end up going dormant early or late.
The growth that you see coming from the bottom, if a lot, suggests that the stems may have dried out and the plant is putting on brand new growth to replace them. However, wait a while. I would hesitate to prune them this early as sometimes they leaf out pretty late on Year 1. They can leaf out as late as the end of May. So live with the dried out stems a while longer and prune them at the bottom if nothing happens by mid to late May.
If it bothers you to see these dried out looking stems, you could also start pruning them in 1 or 2 inch increments. Stop as soon as you see green or when you get to the bottom. Caution: most blooms on hydrangeas appear near the end of the stems so this technique could result in you cutting some flower buds.
You can prevent the stems from drying out if you apply winter protection techniques. For example, some people put chicken wire about 3-6" away from the sides and top of the hydrangea. The more distance, the more protection. Then they fill them up with dried out leaves in the Fall and cover with something that will hold the leaves in place. Excess leaves are stored elsewhere so you can add more in mid-winter due to settling. If packed tight and the stems' ends are protected, the stems should leaf out and produce blooms in April/May-ish.
Technically speaking, you usually do not need to prune hydrangeas. If you have planted the shrub in a location where it can grow to its estimated size at maturity, you will not need to prune often. Maybe every 3-5 years you may need to tweak things. I prefer not to prune although there are some times when it is a good idea. For example, some people reduce the number of blooms so the remaining ones will be bigger or will not cause some stems to bend when wet. Me? Well, I am too busy and just let them be.
Because ES is a rebloomer, you can prune stems at any time of the year. But I would hesitate to prune when the plant has started the broccoli stage and the flowers are fixing to open soon. On a non-rebloomer mophead, I would suggest pruning weeks after the plant has bloomed (so you enjoy the flowers) and before the end of June. June turns out to be the time when I go out and check if I want to or need to prune the hydrangeas.
So feel free to deadhead (not the same thing as pruning) your blooms as this will the trigger new bloomage faster than otherwise.
For additional information on pruning, go here: https://www.hydrangeashydrangeas.com/pruning.html
Does that help you?