Depends on what your personal talents are + what your interests are + demand in the marketplace.
Generally, compensation will be directly proportional to any particular thing no one else can and/or does not want to do and therefore individuals/ society will be willing to pay you to do that thing.
It is critical to understand the above + on top of all of that one must be self aware and know their MBTI, personal limitations, etc.
I know someone who was burnt out in their early 30s in their given career and so they determined to get into medical school and become a surgeon. There were lots of obstacles but they made it by their early 40s. This person nearly missed out on their real calling in life.
Might I suggest:
start from scratch. Be sure and remind yourself that its sort of a big deal to get into a second career at 40 therefore there must be much thought and time and effort put into the ultimate decision. Probably don't want to be starting over at 45, 50, 55. Traction is important.
Start from scratch. An ideal starting point would probably be to get ahold of the list of all possible occupations (US dept of labor website is a great source), income, lifestyle, nature of the work, etc and start the process of elimination.
Jenny is right about resources: be careful to plan within the given limits of reality.
