A fatal error (Not space related) Looks to be 500 error but apache is working fine (no replies)

A fatal error (Not space related) Looks to be 500 error but apache is working fine.

Details
Apache running
Mysql running
Piwik pixel (js) recording data luckily
HTML loads fine on server
PHP loads fine on server

piwik main page /index.php does not work

"A fatal error
Please contact system administrator"

Looks to be 500 error but apache is working fine, any ideas?

Thanks!