# apt-get -f install
See if that tries to fix any problems.

# cat /etc/apt/sources.list
Post that and possible any files in
/etc/apt/sources.d/

# apt-get install unzip
This is just an arbitrary example, see if you are able to install anything from CLI.

# apt-get update
Will attempt to communicate with your repos. Perhaps there is simply an issue with your repo list.

This is just a diagnostic approach to your problem.

If you follow these examples and/or provide more info, then perhaps we can provide a more specific answer.

If you are unable to solve your problem (it seems very doable to fix whatever ails your package manager) then you should consider backing up your files and list of installed packages, settings, logs, etc and then reflashing. But only if it comes to that.