Oh, we all feel that way at times. Different things hold us back--fear of failure, procrastination, the feeling that everything that needs to be said has already been said.
When I feel like that I try and remind myself that writers are not some magical, rarefied creatures (like unicorns) that breathe different air and are not even on the same plane as me. I remind myself of something I once read which was basically that writers are just like you and me; the only difference is that they take the time to write down what they see.
More practically speaking, here are some the things I do to ensure I don't "run out" of writing ideas:
1. Keep notes of potential topics--I use a hidden page of my blog, my phone, and random scraps of paper. A lot of my ideas come on the drive to school, so I am always prepared to capture them before they are gone.
2. I read back over other things I have written for seed ideas (a small part of a published piece that I have more to say about).
3. I daydream. Sometimes I just have to sit with the quiet for a while.
4. I look at art, listen to songs, read--for inspiration.
5. I read the news. So easy to find something about which I have an opinion.
6. I start with my passions (animals; books; teaching; gardening). I could write tons of entries about any one of those, each with a different focus and in a variety of genres, both fiction and non-fiction.
7. Sometimes I just have to sit down and try, even if all I write at first is, "But I've got nothing to write about." Just the act of writing usually prompts something else.