Vodka.
More helpfully, go to the gym, take a break, try and work out what the problem is. Sometimes the plot point or character or whatever just won't work, and needs rebuilding from scratch - just doesn't make sense, or there's no way they would react like that, or something needs re-working utterly. Othertimes it can be nudged back into coherency with some work, so bashing through and getting something out, even if it's not great, then it can at least be refined somewhat into something better. As an on-going way of passing the time, it's also fun to mentally throw characters into different scenarios, and see how they react, and that can then be fed into the actual writing.
More helpfully, go to the gym, take a break, try and work out what the problem is. Sometimes the plot point or character or whatever just won't work, and needs rebuilding from scratch - just doesn't make sense, or there's no way they would react like that, or something needs re-working utterly. Othertimes it can be nudged back into coherency with some work, so bashing through and getting something out, even if it's not great, then it can at least be refined somewhat into something better. As an on-going way of passing the time, it's also fun to mentally throw characters into different scenarios, and see how they react, and that can then be fed into the actual writing.