One approach to writer's block is to simply write: I don't know what to write. I don't know what to write. I don't know what to write... until you get some better idea. Personally if I'm going to write one sentence over and over, I prefer this one: "The quick brown fox jumps over the lazy dog." It includes every letter of the alphabet and is an example of a pangram or holoalphabetic sentence.
Here are a few other pangrams:
Pack my box with five dozen liquor jugs.
Jackdaws love my big sphinx of quartz.
The five boxing wizards jump quickly.
Bright vixens jump; dozy fowl quack.
Quick wafting zephyrs vex bold Jim.
My faxed joke won a pager in the cable TV quiz show.