Unable to Access plugins.php – Server Restrictions?

Hello, I have a brief concern regarding my current site that I am hosting through WordPress.

As of now, my site is being hosted by the web host Verio, and in addition to them I had to hire a SEO service who also does some moderate WordPress management.

My main concern is that it seems like there have been restrictions applied to certain pages on my WordPress, (mainly editor files), like anything relating to plugins or appearance files. For example, whenever I try to access plugins.php I continuously get a message stating that no data could be received through the server… So, is this a root administrative issue?

I also suspect that this error could also be related to a template/theme issue, but I am still not sure about that because my site seems to be running fine as of now.

So, in any case, how should I proceed with this issue? Is there anything I should try via WordPress first, regarding administrative/user permissions?