Unfortunately, there is no simple answer here or easy fix. It is an alignment issue caused by improper use of the USB cable when charging. There are no mechanisms (either physical or software) on the PlayBook that will suddenly not let you charge the device if proper use was followed. If you feel that rebooting the PlayBook fixed your device, go for it, but I can assure you that it was coincidental.
Something was either inserted wrong or is now bent, broken or limiting the normal action of the cable. If you have tried multiple cables without fixing then you can bet confidently that it is the USB port itself. The only solution at that point is to call technical support and see if you qualify for a hardware warranty RMA and send it in for repairs. Trying to fix it yourself is only asking for problems in my opinion and will definitely invalidate your warranty if you snap a pin off.
When looking at the cable from the "male" charging end you will notice that it has angled corners. These are always to go facing upwards/facing the ceiling when inserting the cable. The "teeth" on the bottom side go downward - always.
If the angled corners face downwards/the ground when inserting the cable then it will cause damage to the port. On an OEM BlackBerry microUSB cable, the BlackBerry B logo will assist you visually and will face upwards when inserting.You can think of it this way, The printed Blackberry logo on the actual bottom bezel of the PlayBook will be closest to the B logo on the microUSB cable - they "match" in a way. If they don't match, you're doing it wrong and risk damage to the port/cable.
If using a non-OEM cable then you will have to check what visible marks represent what is upward facing.
