What the diodes supposed to do:
Bypass diodes were designed for solar panels back when most solar arrays were constructed by feeding about a dozen series connected panels onto a grid-tied inverter. The purpose of the diodes is to improve the way a series connected array performs under partial shade conditions. When one cell or row of cells is shaded, the diode across that shaded section bypasses the underperforming cells, allowing the panel to maintain the same current as the other panels in the string. By maintaining the same current, the shaded cell(s), column(s), row(s) or panel(s) to not reduce the output current of unshaded panels.
When a section of a panel is bypassed, the panel output voltage is reduced. This is fine for series connections because the power from the unshaded cells will contribute to the total power produced, but in a parallel connection, this power would be at a voltage that would be too low to charge a battery.
Where it can go wrong:
Sometimes when a solar panel is carelessly connected to a battery bank with reverse polarity, the diodes can burn out and become dead shorts. When this happens, the open circuit voltage of the panel is reduced proportionally to how many diodes are shorted. For example, if you are only measuring about 11V on your panel you probably have half of your diodes shorted. If you are getting 0V, you have all of your diodes shorted.
Since most of our installations use parallel connected panels, and since bypass diodes don't help the performance of parallel connected panels (and since you already voided your warranty by reversing the polarity on your panel) you can repair the panel by simply clipping the diodes out.
Clipping the damaged diode(s) should restore the panel back to full voltage. Although it isn't necessary for parallel connected panels, you can buy replacement diodes and solder them back in. But, before you do this, be sure to take a picture of the open junction box so you can see the way the diodes should be installed. If the line at the end of the damaged diode is on the right side, your replacement diode better have it's line pointed to the right as well. Diodes aren't like resistors, they have polarity and can be installed backwards.