You rebuild a file when there's corruption on the file.

1. Make a backup. 
2. Hold Windows key + E on your keyboard to launch file explorer.
3. Go to you local c drive and open the Pastelxx/Xpressxx folder.
   • Where xx represents the version of Sage 50cloud Pastel you're running
   • Example: Pastel19 or Xpress19
4. Double click Rebuild32 and click OK on the message.
5. In the small box, select the drive where your company folder is.
6. In the second box, double click the company folder.
7. In the last block, select the relevant files and click Rebuild. 
8. Ensure all records are recovered by clicking View.
   

 CAUTION: Restore an older backup of before you received the original error if you receive a Critical Error 2. Reprocess any missing transactions. 

Refer to article Restoring a backup.