The client may accidentally choose another template or reset existing template to default one in builder. Note, that this action removes current client's project. However it still can be restored in 2 ways:
Open builder for this website, expand "Publish" menu and choose "Backup/Restore" option. Switch to tab "Restore". There you may see some auto-saved backups of earlier website state saved. Try choosing the oldest one and click "Restore" button. After that check if it has been restored.
If website cannot be restored with option (1), but you have correct live published website, then you can restore website in builder from published website. Take following steps:
clean client's folder in builder which holds client's current project data/e/x/example.com/ (where client's domain is "example.com" and part "/e/x/" is 1st and 2nd domain's letters)
copy folders "sitepro/dat" and "sitepro/gallery" into client's project folder in builder, so that copied folders are located in:
data/e/x/example.com/dat
data/e/x/example.com/gallery
update permissions of copied folders if needed
After that you should see last time client's published website in builder when reloading it.
Note that even if the spoiled website was published and there is no live website remained correct, then you may try to firstly restore client's account on hosting to an earlier state where the live website is correct, and then restore website in builder following option (2).