First of all – don’t panic!
All things being equal, it will be one of your plugins that is causing the problem – providing your site was working well before.
Log into your host server using FTP or SSH if you are really lucky.
Navigate to the root of your website, then to the htdocs, then to wp-content.
Move the existing plugins directory to plugins.old (mv plugins/ plugins.old/)
try your site again – if it works then you have narrowed the problem to a plugin! – see the bottom if not.
Create a new directory called plugins (mkdir plugins)
Now move the plugins back one by one (cd into the plugins.old directory then mv plugin_directory_name/ ../plugins/plugin_directory_name/)
After each move – check if the site is still up
if it is then continue with the next plugin
if its down – put the last plugin back and move the rest in
Just keep going till all the plugins that are working are back in and the troublesome plugin is still in the .old directory
Then find an alternative plugin for your needs
All ok in the end?
If you need a hand – contact me!