This happens to everyone. Others have written some excellent advice re the whys and wherefores of reduced motivation, and I won't go there. I prefer some practical advice.
Change your subjects. Often as not motivation wanes when we repeat the same motions and revisit familiar places to reshoot old subjects. Think laterally, move sideways.
Forget beautiful images, forget perfection. Go with "good is good enough". Step out of your comfort zone and just shoot from the hip. I did this in the early 1990s when I had to face a series of personal setbacks and my motivation in general went downhill for about a year. Regular bush treks and city walks gave me an opportunities to escape from my problems and look at new things. Most of what I shot (in B&W) was processed and filed away. Now I'm retrieving those photo files and preparing to print them in a series of small booklets. I'm surprised at how good many of those images are, but then I would think that, being me.
Go one camera, one lens, one film for a while. My favorite combo is a Nikkormat FT2 with a 35mm f/2 'O' lens, a yellow filter, and fast B&W film.go walking and shoot whatever catches your eye. This connects well with the previous point. In 2012 after I retired I spent two weeks in Melbourne, walking the city streets and just photographing high rise buildings in different angles from the streets and sidewalks, also whatever I happened to see that caught my interest. The results have formed a collection I think is "interesting", I'm not sure what if anything I will do with them but they stimulate me to look at them and remember those times.
Give up photography for a while. take up reading good books, write a novel, do a project at home, revamp the garden, design a house. Whatever. As long as it's out of your comfort zone (= rut).
Go away somewhere. Take a trip. This can be a weekend or a few days away from home, or overseas. I regularly travel to Indonesia, Malaysia Brunei and Sarawak to find and photograph colonial architecture, which gives me no end of stimulation (and a fair few unexpected adventures) along the way. Yes, I know it has all been done before, but not by me (or by you)..
Do some scanning. That always rejigs my interest in getting out and shooting, if only to get escape from the scanner and the essentially dreary process that is scanning.
If you have a darkroom, choose some old negatives and reprint them in new ways. Think small. Do only 4x5s or 5x7s. During my last bout of not wanting to do anything photographic, I assigned myself a project of printing some very old snaps of family and friends in a new size, 'half three by fives' (I have an old multi-frame easel with a frame for this old 'contact print' size) on old FB paper I've had lying around since 2003, and then selenium toning them to a really neat reddish-brownish color. These were then given away to people. One friend has four of them in a beautiful frame on his study wall. I have to say it looks quite superb.
Visit local charity shops or photo stores (if you can find any) to see what secondhand cameras they have in stock. Don't spend a fortune, buy something you can play with and then either give away or flog off on Ebay.
The possibilities are seemingly endless if we think laterally. The only problem is to find the time to do all these interesting things
And do let us know how it turns out for you.
I especially enjoyed ced's post (#6), to which I will add one of my favorites: if at first you don't succeed (in photography or anything else), you're batting about average.