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Or try rewriting something short; a fairy tale or joke. That way you don't have to worry about characters or plot but you can stamp your 'voice' onto the story. Should help exercise the old 'writing muscles' but I'm not sure if they are located in your head or hand?
How about keeping a journal?
It doesn't matter if it includes to do or shopping lists. It might be about writing of the things you did during your day even if it is "I didn't do much today except taking a shower, having a meal and brushing my teeth"
Personally, this happens a lot to me. I want to write, but I don't always have the motivation to put it to paper once I can actually sit down.
I don't think it is your imagination going away, but motivation and just the general timing of your inspiration. Inspiration (in my opinion) comes and goes so quickly that you need to find something that will help you jot down that quick thought or idea.
If you have any type of smartphone or tablet, it might be easiest to find an app like a notepad in order for you to jot down that quick thought when it comes. If you (like me) don't have a smartphone, carry around a little notepad and pen. They are really nice for just keeping thoughts in. Then, at a later time (and once you are free to write), you can come back to the ideas to see if your inspiration and motivation are still waiting for you.
Just remember, if you find that the inspiration and motivation has gone, don't be too sad. Both are finicky as is, and forcing yourself to write when you don't have the proper mindset, time, and place to write is hard (and it makes your writing sound forced). As it comes and goes, just let it pass through your head (and your smartphone/pen and paper if you have it).
Elyon's Love and Strength,