My dryer had a 4 prong plug, and I moved into an apt and the outlet is 3 prong I changed the plug to 3 prong and now my dryer doesnt work

I would start at the breaker (which I'm sure you 've already checked). Then if you haven't already, I would check the voltage at the outlet. For this you'll need an anolog voltohm meter or a digital multimeter. You can get the anolog version for under 20 dollars. I would go with it if you choose to do this, especially if you're not going to be using it a lot.
Dont worry if you've never used one, because I can tell you how to do it.
First take a look at the dryer outlet. You'll want to make sure it's getting the proper voltage. You should have an "L" shaped slot. That one is neutral. The other two should be rectangular shaped. Those two are the "hot." This configuration is the most common.
If you do choose to get a meter, all you have to do is set the dial to the right place, and read the display. It should have a red test lead and a black test lead. If your dryer is an electric dryer, you'll be measuring for 240 volts AC. On the meter you want to set the dial to the next highest number above 240 (probably 250). here's what to do:
1. Insert a test lead into each rectangular slot. You should get a reading around 240 volts.
2. Insert one of the test leads into a rectangular slot, the other into the "L" shaped slot. This time you'll want your meter a step above 120 volts AC. The next highest number. Take the reading, you should get around 120 volts.
3. Next do the same thing as number 2, just on the other side. (rectangle slot to neutral L shaped slot). You should also get around 120 volts.

If..... You dont get a reading for the 240 volts on the first test, you'll know that you're not getting power at the outlet.
Also, if one of the "sides" for the 120 volts shows 0 or a number thats really low, then one of the legs of the circuit breaker is tripped. When you check the circuit breaker, make sure that you turn both of them off, and then on again. You see, it uses two breakers (or fuses) that supply 120 volts to both sides of the plug, thus giving the dryer 240 volts of electricity.

Also make sure that when you hooked the 3 wires up to your dryer, that you placed the metal strip at the metal terminal and secured it with your middle wire, at the middle terminal.

You could also remove the cord and do a continuity test on it to make sure that it's not the problem. On your meter, you can use the ohms setting, set on the lowest one.
Just like this. It's usually RX1, or RX1K. If you look up at the upper left hand corner of the meter, you'll see the ohms sign. Some have a thumbwheel, that when you first turn it on, you touch the two meter leads together, if it doesnt go to 0, you use that wheel to adjust it to zero.
You can also see on the upper right hand side, the AC portion of the voltage test. You would use the 250 setting on this one to do all of the tests above.

I hope this helps you. I will be more than happy to help if you need any further assistance.
Thanks for your time,

Mike