Welcome to the wonderful world of the pedia's and these forums. I will try and help you with your problem.
Pocketpedia was not really designed as a backup restore source but it can be with a bit of work.
First off these are the Folders and Files that should be in ~/Library/Application Support/cdpedia/ .
Covers
InfoTemplates
Plug-ins
Templates
AddMultiple.xml
autofill.xml
backup1Month.cdpd
backup2Months.cdpd
cdpedia Backup.zip
Database.cdpd
Database.cdpd-shm
Pocketpedia.cdpd
Pocketpedia.cdpd-shm
Pocketpedia.cdpd-wal
The above entries in blue are Folders do not delete them or their contents.
AND of course do not delete: Database.cdpd OR Pocketpedia.cdpd
The Files with the suffix; .shm & .wal can be deleted as they are changed and recreated automatically again on each starting up of your pedia. You can think of them as a temporary index cache files. The Database and Backup files are self explanatory so not advisable to delete those. The .xml are again a type of index file for your autofill fields and are important so do not delete those two. You can clean these up to some extend within each Pedia's Preferences > Autofill.
1 - The best restore method is to use the data folder. If you have access to a backup of your mac drive, as everybody should, restore ~Library > Application Support > CDpedia without CDpedia running. That folder has all your data for CDpedia. Data and Images.
2 - If however it is only the Database.cdpd that is missing and you do not have access to a backup of your mac drive BUT your ~Library > Application Support > CDpedia is relatively still intact you should see the file; backup1Month.cdpd With CDpedia not running, double click on this file and CDpedia will open and import that backup data, remember it will be a month old.
3 - If backup1Month.cdpd is not present look for the file; Pocketpedia.cdpd If you do then: Without CDpedia running, change the name of Database.dvdpd to DatabaseOLD.dvdpd if it exists and rename Pocketpedia.cdpd to Database.cdpd. Now fire-up CDpedia and you should have all the data from Pocketpedia, without the covers.
4 - To reinstate your missing covers: Whilst in the List View select from the Menubar Edit > Select All, now Right Click in the Cover Well on the bottom left and select Download from and choose your preferred site. Doghouse being a good choice first off.
5a - Finally as a last resort because ~/Library/Application Support/cdpedia/ is missing or empty, you can carry out the following (though I have not tried this as I have multiple backups all over the place), you can restore your data from an iPhone backup as well. Pull the data directly from a backup with Backup Extractor.
5b - Launch it and press the "Read Backups" and select the device and then look for com.bruji.pocketpedia3. Select it and use extract to your desktop. Your looking for the same Database.cdpd file that will be in Private Documents folder extracted. Move it to the data folder (~/Library/Application Support/CDpedia) as mentioned above (actually you can double click any .cdpd file to make it the database, but manually doing it, you know what is going on as well).
Always remember the 3 rules of computing: BACKUP, BackUp, backup. Apart from macOS own TimeMachine backup facility there are 3rd party programs available. I would recommend Carbon Copy Cloner (which I have been using for years for my off site backup)
Also the pedia's have a couple of ways to backup as well:
a) The CDpedia makes copies on a periodical basis. It's called Backup1Month.CDpd
b) You can also do a manual backup yourself from within CDpedia: Select from the Menubar File > Export Collection > Backup
I hope one of the above you have success. Please post your success or failure explaining what worked or did not work.
Cheers