Hi there, & Welcome to Tom's Hardware!
Glad you figured it out. Always a good feeling.
Some of the newer wireless combos don't have a battery indicator warning so they can just stop functioning until they get a fresh AA cell.
Other thing to remember is that you can go to the Control Panel (Mouse or Keyboard) and check the hardware tab. A windows update or other new app could switch that combo to a different default device so it might not function properly until switched back.
Always good to have a 'legacy' mouse and keyboard handy to swap in when there's any question. I have one of the first IBM wired keyboards that audibly and physically makes a click, I still enjoy using for testing, or new computer startup.