The mind has a wonderful resilient nature. I, too, find myself with a lack of imagination sometimes. Mine is a self induced job problem. I work in IT, not an occupation known for it's creativity...I'm just saying.
So, in order to combat this, I have started doing a few things:
1) Keep a pad and pencil by the bed. You are correct, your mind comes up with some wild stuff during dreams. When you have a good one, write it down. I often use this as brainstorm beginning.
2) Brainstorm. Take a tought and follow it out, writing down whatever you can come up with. If it's a simple topic, like the President or the economy...write down how these things make you feel...then examine those words. You will be amazed at how fast your mind will begin to understand what you want it to do.
Your imagination is not permanently lost. But, it is in hibernation. You can wake it up, and by keeping trying, you will.
Every single one of these thoughts are great and should be thought about, in my opnion. Personally....I will taking afew ideas from here as well.
You are not alone in this type of issue. The question, really, is....what do you do about it?