There are many things you can do to kickstart motivation, but many of them are going to depend on who you are as a person.
Here are a couple things that I have found that work for me:
- Working on another project and only thinking about the project that I was burned out on.
- Taking an extended vacation.
- Getting involved in a community that is related to the idea that burnt me out.
- Reading a motivation book, such as How To Become CEO
- Watching a biopic from an individual I admire, such as The Aviator (Howard Hughes).
- Change gears with the idea. If you were programming, try doing market research. If you were writing a business plan, try programming, etc.
It is different for each person, but you have to find a way to re-ignite the fire. On the flip side, you also have to know when to let go.