Can printed circuit boards be ruined when placed into current incorrectly?

Hello, community!
So I am very new to electronics and this is a basic question. I was looking at what some people are asking on this forum and this is kind of embarrassing, but I am curious to know. I was taking apart a toy that has a green circuit board with quite a few components on it and connected to it. The lights are programmed by the system, just so you're aware. I removed all the plastic and junk out of the way so I was just left with a green wafer board with LEDs attached, amoung other components. I cut off a microusb tip and exposed the positive and negative wires for my power source. I then connected the red and black wires from the green wafer to the usb power source and I got the toy to light up! It normally runs off AAs but I got it to run off usb, kinda cool. As I mentioned above, the lights are programmed/told by the green wafer board to light up in a specific repeating pattern, so when power is supplied to it, they don't light up all at once constanly, they are timed and varied. Anyway, thats not the topic of my question. The power being transferred to the little green wafer didn't seem to damage anything as I have connected it many times, successfully. But then last night I experimented by connecting the green wafer into the circuit backwards. So picture this. I have a mini breadboard and I ran a current through the bread board with usb. Now, previously I plugged the green wafer into the breadboard in the correct direction of current (current flows from positive to negative, so I normally would have connected it the same way) and it lights up fine. Lastnight, however, I reversed the wafers direction in the current so that the negative wire from wafer was head on with positive current in breadboard. After nothing lite up, I reconnected it the normal way and immediately after the light started flickering weakly as if something went wrong when I reversed it (clearly it did), and it barely works now. Remember, I started by connecting the circuit board the correct way to make sure everything was working fine. Then I connected it the wrong way and put it BACK to correct way, it didn't work right (as I mentioned it flickered weakly and now doesn't work at all).  Did it "fry" or something? I don't care about the wafer it was supposed to be an experiment, I just want to know what happened from an engineering/physics perspective. Why isn't it worming the same and what happened when I reversed the green wafer in the current? I mean LEDs, diodes, when reversed, just dont light up and if you put them the proper way they should work fine..even if they were put in current incorrectly. What is different with a printed circuit board that ruins or prevents it from performing how it did before reversing it? I've attemped researching this a little, came to the term reversed polarity, but I still don't understand why the system has been damaged? I am 1st year physics student so please use technical terms that I can google or may already know.
Interestingly, afterwards I pulled the LEDs off the wafer board and connected them into a circuit with a resistor and they work fine, so I can deduce from this the lights are fine, its definitely the printed circuit board. 
Thank you for taking the time to read!
J

Yes, you have ruined the board. Guess you can call it a lesson learned.

Reversing the polarity on a board which does not have reverse polarity protection, which is typically  a diode  or a P channel FET, will often cause some damage. LEDs will tolerate low reverse voltage, particularly at 5V, since they normally have a series resistor. However other devices, particularly Integrated circuits, will often not survive. Most Integrated circuits have an  internal diode structure on their inputs and output pins, which are in series with the power rails. When the voltage on the chip is reversed, there is little to prevent a large current flowing in the diodes, destroying them and their associated circuitry.Electrolytic capacitors are also sensitive to reverse voltage. In some cases, if the current that flows in the reverse polarity condition is high enough, the evidence of damage may be visible, an electrolytic may have lost its case, an IC may have lost plastic off its case, and wiring tracks on the board may have melted. This is not likely in your case.Without knowing what parts are on the PCB, it is difficult to give a definitive answer, but the simple answer is definitely yes,cheers,Richard

There's a well known joke in electronics circles that connecting things with the wrong polarity "Lets the magic smoke out".  This is the magic smoke that makes the device work, and if it is let out it is very difficult to get it back in again.  If you see the magic smoke, your device is usually dead.  But you don't always see it...