Like the great wolf pack a wheel is only strong once all of the spokes are strong. A broken spoke makes a wheel weak and if left alone will inevitably cause the others to weaken and break. Spokes break from time to time, but if you catch and fix it early it's not the end of the world. You can usually ride on the wheel for a few kms to get you home without doing serious damage but don't ride on it for days as it will only get worse. (Remember, having no bike means a perfect time to bring your dog for a walk.)
What To Do With A Broken Spoke:
Spokes tend to break through general wear and tear or if you hit a pothole too hard. If you start to feel a wobble in your back or front wheel then give it a quick inspection. Grab on to your spokes and give them a pull to test there is no movement. A broken spoke will be quickly identifiable.
If you can, unscrew the nipple (small screw head that fits to the wheel rim) and take it out. You can do this with a spoke key (pic) but if it is loose, you may do it by hand. If you cannot remove it or do not have a spoke key, tape it back in place so it does not get stuck into the wheel when it is moving. If you have a number of broken spokes then do the same
Replacing a broken spoke is a little more tricky as you will need a few different tools including, the correct size spoke and nipples, a wheel truing stand, spoke key and a lot of patience. If you're not ready to learn then drop the bike into a mechanic and get it sorted. If it's just as single broken spoke on a generic wheel then it can be relatively cheap to replace.
Thanks for reading! Einstein is out - woof!