Hi Sylvia,
Thank you for writing in, did you install or update something before this issue happened? If you install/update a plugin when this issue happen, try deactivating that plugin. Since you cannot login to your WordPress dashboard due to the 503 error, you will need to connect to your website via a FTP client or File Manager in cPanel. Once connected, go to /wp-content/plugins/ and look for the folder of the suspecting plugin, rename it to something so it will be deactivated.
If its fine to deactivate all the plugins, then rename the plugins folder (in /wp-content/). Then check if that resolves the 503 error.
If it does, then activate your plugins one by on from your admin dashboard, and see which one will caused the 503 error.
If that does not resolve the issue, please provide us login credentials (wp-admin and FTP) in a secure note so we can take a closer look.
Let us now how it goes,
Cheers!