When those situations arise, I tend to ask myself five or so simple questions to get my creative juices flowing once again. Since I highly doubt I'm the only one who suffers from the occasional case of writer's block, I thought others might find these questions useful as well.
1. What is Your Character's Concept? This is easily the most important question to ask yourself when creating a new character. The concept will act as a blue-print for the character, influencing almost every decision you make during the creation/generation process and beyond. When creating the character's concept, try to boil it down to its most essential elements and create a short and memorable phrase. While you could write an entire paragraph about your Fighter's concept, it will be easier to remember that he's a "Runaway Farmer's Son Looking For Adventure" and its much easier for others to get what your character's concept is as well.
2. What Are Your Character's Major Beliefs? All of us have our own set of personal beliefs that shape our decisions and life. It only makes sense your character would have these as well. While you don't have to get very detailed with them, its a good idea to at least ponder what your character's opinions and thoughts might be on things like religion, politics, and how one should live their life. For example, our "Runaway Farmer's Son Looking For Adventure" might believe that a person must work hard if they want to achieve something and if you want something done, you should do it yourself due to his childhood working on a farm. Because of those beliefs, he might have a problem with a noble who is handed things they didn't earn and has others do their dirty work for them. Your character's beliefs should influence how you play the character and how they might react to certain situations.
3. What Major Events Shaped Your Character's Life? All of us have had a number of experiences that helped shape who we are today. That should be true for your character as well. Maybe there village was attacked by a tribe of goblins and they were the only survivor, instilling in them a deep hatred of goblins. Maybe they were a criminal, but after spending sometime in prison they have decided to turn their life around and make up for their past ills. While you could detail all the events in your character's life, I suggest focusing only a handful of major events. All those extra details can be interesting, but you don't want to get bogged down because of them.
4. What Are Your Character's Major Goals in Life? Throughout life, we all have a handful of goals that we wish to achieve. You character probably has a few goals as well. Maybe the character who's village was destroyed by goblins hopes to one day find the tribe responsible and get vengeance. Maybe your swordsman is trying to find the man with an extra digit on his hand who killed his father. Having a few goals will help explain why your character is living a life of adventure and will give your Game Master a few hooks to use in the future as well.
5. What Are Your Character's Qualities & Flaws? Think of this question as you adding the icing to the cake that is your character. Think about your character's concept and everything else you have decided and create a few positive and negative traits that would make the character a little more interesting. For example, you have decided to play a Paladin who used to be a thief until he saw the error of his ways and took up the sword to pay for his past crimes. Your Paladin might be more sympathetic to those who are trying to redeem themselves due to his own past and he is excited to do anything he can to help others. However, due to his past, he might struggle when put into a situation that might cause him to buckle and return to his criminal ways. These little details just add a bit more life to a character and shouldn't be skipped over during the character creation process.