As far as contractors and installers go, the major old-school, single-paned window fix is new windows. There are double and triple paned windows out there and a wide variety of costs. You’ll have to do some research and then, call around. Or, you can leave your data at a website and they can spread your information around. It’s up to you!
Double paned windows have either trapped air or trapped gas (krypton or argon) between two panes of glass. Sometimes, the glass is treated with a UV filter. They come pre-framed in aluminum, vinyl and fiberglass, and for loads more money you can get them in wooden frames. The trapped air acts to slow down the transfer of heat from one pane to the other making it “insulated.” Because of that, you’re less likely to get fog or ice crystals on the inside or the outside pane.
I had some installed and they actually had to make the window hole larger and lower to the ground/floor to follow new fire laws which say something like: the window must be accessible to an averaged height firefighter in case ingress/egress is blocked at normal exits. I’m just sayin’. Be prepared in case that’s a real thing. It hasn’t hurt the way the house looks or how the windows work.