external hdd show up in My computer, but not able to go inside (HDD size not shown as well), how can I solve it?

Hello,

The issue you're experiencing could happen due to insufficient power going to the drive ,USB cable or port gone bad. You could try different USB ports, different USB cables and on a different machine. When changing the cable make sure the new one is as short as the original one, or even shorter. This is important it terms of providing enough power for the drive to function properly.
I would start with running the manufacturer’s diagnostics tool to check the drive health. IT this is ok then I would check the status of the HDD in Disc Management. To open Disc Management:

1. Right-click on the Computer icon on the desktop or in the Start Menu.
2. Left-click on Manage.
3. Left-click on Disk Management.

The next step would depend on the HDD status displayed there.

Hope that helps and don't forget to come back with more info.