I have been gone a few weeks. Now my mint has all flowered. Can I still use the leaves? I want to make a batch of mint jelly.
Mint leaves are just fine to use any time, including after the plant has flowered. The flavor may not be quite as strong as it was before it flowered, so you may need to add more leaves to your jelly infusion to get the same taste. Be sure to cut the flowering stems back when you harvest. Cutting the flowering stems back may even encourage your mint to put out a new set of fresh leaves for later.
Mint leaves can be used at any time when green, but they have a stronger flavor before flowering. This is true of many related plants. I usually let my mint flower, to attract bees and butterflies, and remove the flowerheads before they set seed, but you can cut the flowers at any time. If your plants seem to be developing seeds, cutting the plants back further (than just clipping off the flowerheads) may help coax out a new flush of growth.
As TeresaMcgH points out, because the mint has flowered, you may have to use more mint than usual for good flavor.