When your WordPress website is down (or any website for that matter), it may not be because of a hack. If you suspect your site has been hacked, do a full backup. Now check with your hosting provider and see if there are any network outages or server issues which could result in your site not being online. You may need to raise a ticket to get a response from your web host. Now it depends on whether your web host are helpful or not and also your technical ability. On 1 occasion when I was brought in to investigate a hacked WordPress website, the web host investigated and claimed it had not been hacked due to the fact the website was operating as normal for ‘normal users’. However it had a back door to serve unsavoury content which they could not see as they were a ‘normal user’.
Hopefully you have an up to date backup. Be careful though, the hack may have happened before your latest backup so attempt to find out when the hack occurred and load the backup prior to this date.
Once the backup is restored; check your website. Hopefully your website is up and running again.
Ideally it is worth trying to find how the hack occurred. Of course, if you reloaded from backup, any traces of the hack will be gone. However, the backup that was done once the hack was suspected can be restored to another server and investigated although you may need to get an expert in this area to do that.