Missing the part above the electric plug-in

If I am not mistaken, and without any visuals, my best guess at this time would be that you are talking about the cover for the serial port. It is located directly above the charging socket. this should not have anything to do with your Roomba not charging. If the light on your charger is not coming on at all, try a different wall outlet. Also, use a multimeter and check the output of the charger, it should be 22volts. For the Generation 1 and 1A : If the power light is off or flashing, discard both the Home Base and the Power Supply and replace the main pcb on the Roomba. This is the suggestion of the service manual, because a flashing light power supply can destroy the PCB. If your power supply is working properly, start by taking a closer look at the battery. Batteries do degrade with time, and it might be possible that you need new one. Hope this helps to get you stated. Let us know how it is going. Good Luck