This has happened to me MANY TIMES.
Usually, I start my writing process full of enthusiasm and have so many ideas about it in my head: a whole plot for my 200 000 words novel and many scenes are ready to be put on the paper. But, after a few weeks or months and 10 000 words (which I’m proud of) I run out of it.
To this day (I’ve written stories / started novels since I was 8 years old), I haven’t find a solution. BUT, I think I’m now realising something…
Maybe, you should take a break from writing (if you just can) and do something else. Go out and spend time in different inspirational places or meet people and listen what they have to say OR read things widely / watch videos and find inspiration from internet. Why? It will give you new perspectives which could possibly inspire you to write again.