Often enough, with low cost power bank, tendency to damage and loss connection between micro USB and power bank renders the unit in-useable.
An easy method to salvage by using any USB cable is worth the time spent to rework it.
Step 1: Open and Remove the Damaged Micro Usb
Open up the power bank and remove the micro USB
Step 2: USB Cable
Find un-used USB cable and cut away the connector at far end of the USB connector.
Step 3: Solder Cable Onto Power Bank
A bit tricky and need google the pin out. Basically is using only black (GND) and red (VCC) on the USB cable. Cut the shield and other 2 data wires.
Solder the black onto the right hand pad on the board where the micro USB original sit and red wire on the right side.
Step 4: Secure the Cable
Using a hot glue. Secure the GND/voltage with hot glue.
Step 5: Test
If required. Test the voltage to ensure power is transmitting. There you are, all done.