USB port was repeatedly disconnecting but now doesn't work at all

It is not unlikely that you're dealing with a hardware problem here.

1. Solder joints connecting the USB socket to the motherboard may have failed.
2. The USB socket might be worn out. The average USB socket will resist about a thousand connect/disconnect cycles before it starts to flake out, but this may vary according to circumstances. Dust and dirt will make the socket wear out faster, as will undue mechanical stress e.g. USB plugs or devices repeatedly forced sideways, knocks, etc.
3. Power problems. The USB port supplies power to connected devices. Some devices, such as external HDDs, tend to consume more power than what can be supplied by a single USB port - this is why some external HDDs come with a cable that has two USB plugs. USB ports are protected from excessive power drain by solid state fuses, these limit the current to safe levels. However operating the USB port close to (or just over) its power limit will degrade these components over time. When this happens, the USB port might work with devices that use little power e.g. a mouse, but it will no longer work well with devices that require more power such as external HDDs.

Resolving this type of problem requires getting access to the computer's innards, and reasonably advanced skills with a soldering iron.