1. There's a "Preview" button below the textarea. Please use it, your postings are hard to follow.
2. Try to use your brain first.
When a new problem arises, try to fix it for yourself first. So you won't need those "Ah already fixed" anymore.
3. You probably should get more familiar with the process of compiling and linking -- maybe C++ in general, too.
However, your problem is that "Board::restart()" is used and declared, but not defined.