Some blocks are caused by desiring a mathematical goal but being unable to achieve it. Some are caused by losing your desire to do mathematics.

I think the second type of block happens more often than most people admit. It needn't be cataclysmic: it might just show up in a feeling of being burnt-out and tired. You've lost your appetite, your energy, your fizz. Everything's a burden.

When that happens, it can be useful to remember that you don't have to spend your life doing mathematics. However great a mathematician you are, it will make next to no difference to the world at large if you drop math and become a postman. Seriously: you could do something else. You'd survive, and you might be perfectly happy.

Once you've really internalized that, the feeling of heaviness should lift. You're doing mathematics because you want to, not because you have to. It's not an obligation. And if your appetite for mathematics never returns, that's probably a signal that you should do something else.

This might sound like depressing advice, but it shouldn't be. I find it freeing and energizing.