Have you ever felt like no matter how hard you try, you just can't write? More than not being able to write anything, you feel like you can't write anything good? If you're a writer, chances are you've experienced this. In many ways, not being able to come up with ideas to write about is not nearly as difficult to overcome as doubting your own work. How can you power through when everything you write seems bad?
For starters, no matter how bad it is, it's probably better than you think it is; artists are perfectionists and we are almost always biased against our own creation. However, that doesn't mean it isn't poor writing and something doesn't need to be done about it. I think all writers hit that block. Of course there's "writer's block" in smaller amounts, but it also comes in seasons as well. I do not doubt that even CS Lewis, JRR Tolkien, Charles Dickens, JK Rowling and all the great authors we know and love today live through those seasons.
When it comes to overcoming seasons of writer's block, I don't claim to be an expert. All I know is what works for me and other writers I know.
Just write.
Even if it comes out serving better as toilet paper, keep writing. Keep exercising that muscle. That might be all it takes is time. But it won't get better if you're not writing.
Take breaks.
It may seem a bit contradictory to what I just said, but sometimes you do need to take a step back and come back at it with a fresh brain.
Spice it up.
Try to spice up something that already exists. For example, a candy bar wrapper. Look at the ingredients and write a paragraph telling people what's in that candy bar, only try to make it interesting -- really sell it. With something like this, you don't have to worry about flow or if content is meaningful. You're focusing on one particular aspect of writing: drawing the audience in.
Rewrite.
Rewrite a poorly written movie scene in book form. You can either take something already bad and make it worse on purpose, or you can try to make something bad better. You already know where it's going, it's nothing new, you're not writing a new storyline. This time, however, you're forcing yourself to think, "Okay, what makes this bad writing?"
Time yourself.
Set a timer for a minute and just go at it. Even if you're writing the same word again and again until you think of what comes next, just brainstorm. Do not stop until that timer goes off.
Try something new.
If you like writing science fiction, switch it up and try writing historical fiction. If you already write many genres, try a children's story, a letter (real or fictional) or even an autobiography.
Create a story map.
Try diagramming an entire story. If you want an extra challenge to really stretch yourself, start at the end of the story. Think, "Okay, this is where I ultimately want to end up. Now how do I get there?" and work backwards from there. Even if you do not generally like story mapping, this isn't something you have to do with every story, but it's a good exercise to work on flow.
Prompt yourself.
Prompts are beautiful, as I am sure any writer already knows. If you want to take it a step further, write a bunch of short blurbs using many different prompts. Then, try to piece a few (or bunch) of the prompts together to make a story (this is another good exercise for flow).
Change your environment.
Sit outside. Brainstorm hanging upside down from your bed. See things from a new perspective.
Analyze another work.
Read other works and critique them. Whether it's from a friend or a published author, there will always be something to critique. See what can be better in their writing and it will help with your writing as well. You would be surprised how much more you learn about your own work by critiquing someone else's work!
These are just a few suggestions (it certainly isn't a comprehensive list -- there are more tips out there). Not all of these will work for you; maybe none of them will. Don't try them all at once, pick maybe one or two for now and if those don't work move onto a different one from the list.
One more note I leave you with: you will have crappy writing. Just accept it. In one of my classes, a guest speaker once came and spoke on the creativity process. One thing he said that stood out to me was, "You have to be willing to make bad art. Otherwise, there would never be any good art." So go forth and write poorly!