My only advice is to make sure you distinguish between short term and long term burnout. I once left an entertaining startup (that was later aquired by Apple) because I thought I was unhappy with the field, the job, the office, the nature of the work. I took two months off between jobs, because that was the only way I felt I could get time off.

At the end of the two months, I have to say, the old job didn't sound so bad anymore. I started grad school afterwards, so I was charged up, but I suspect that the time off was what I really needed. Looking back, it was actually one of the more innovative startups I've ever worked at.

You may be looking at deep long term burnout, but how long as it been since you've walked away from work long enough to reset your head? It doesn't have to be decades - a single year of 10 hour days where you often find yourself at the office on weekends can be enough to cause your mind to rebel and shut down. Think of your head as an employee that has been pushed too hard, and has retaliated by not producing anything.

I've learned to identify my short-term tendency to burn out and distinguish it from true career malaise. Maybe I just need more time off than the average person. I wish I could drive harder, but if I can't, I may as well know that about myself and minimize the damage.