HACKED WEBSITE???Limited admin access

I am having problems with my website, for past week.
1. I cannot install new plugins, or delete any ( I am still admin of the site, and I have deleted all other admins, that I did not recognize)
2. Cannot delete old themes
3. An additional problem is How to get rid of Checkandgo.info redirecting my site?I am aware this is malicious software, I read that I will need to reinstall WordPress, that requires site back up, and plugins installation which I am unable to do (but I previously was able to do all that)

I am not an advanced user so please be kind

Help, please

The page I need help with: [log in to see the link]