First you need to properly uninstall matlab from your computer.
Find the location of the main matlab file. Mine looks like this:
then follow the below pathway
and find the uninstall folder. Read the .txt file in their for more instructions.
You then should be able to download it again from really any link and then when you run the .exe installer you will be prompted for your account information to validate you own a license.
If this doesnt help, you should describe the issue more. e.g. what error are you getting or what it specifically isn't doing when you go to download it.