Answer: Growing broccoli in the warm weather is not easy. If your broccoli begins to produce yellow flowers you may be in trouble, but not all hope is lost. If your broccoli variety produces side shoots and most do, then you can just cut off the flowering center head and hope for some side shoots.
Now this only works if you still have healthy leaves. If all your leaves are dying off (click here for a picture), then your best chance is to try again next season or plant fall broccoli. Broccoli does better in the fall because it likes to grow in the colder air temperatures. I live in zone 5, so I am planting my broccoli now from seed and expect to harvest it 3 months from now, which will put me in early October.
To learn how to prevent broccoli from bolting, click here.
To learn more about growing broccoli, check out my experience with growing the packman variety this season.