I had to do a restore.
when it was done I discovered that I had all my folders but there was no data in them. where is it and what happened?
Depends which folders.
For Local Folders, so unsynced data that exists only in eM Client, anything that was in the Local Folders before you did the restore will be gone and replaced by what is in the backup. If you want to restore only some Local Folder data, to add to what you already have in Local Folders, you can follow the partial restore described here.
For synced folders, so data that is stored on the server, after the restore they will immediately revert to what is on the server. Essentially the restore has no effect, so whatever was there before the restore, will be there after. No change. If you want to restore synced data that you deleted from the server, if you have a recent backup from when that data was there, you can do an offline restore as described here.
And Data File folders are not included in the backup/restore, so unchanged, though you will have to mount them again after the restore if they weren’t mounted when you made the backup.