What happens when you use the Import button in Picasa?
When you select the Import button in Picasa and choose your camera or memory card under the "Import from" drop-down menu, Picasa begins loading the images from the camera.
To cache the photos and videos, Picasa creates a temporary folder called ".Picasa3Temp"
The ".Picasa3Temp" folder can be found on your computer either on the C drive under "My Pictures" / "Pictures" or in your default downloads folder (depending on how your computer is set up).
The folder has a period / dot as the first character so it will not be visible to Picasa.
The folder is also set to hidden so it won't be seen in Windows File Explorer (unless your Explorer settings are set to show hidden files and folders).
When you exit the Import process or complete the Import, Picasa deletes this ".Picasa3Temp" folder.
If Picasa has importing errors or crashes, the .Picasa3Temp folder may fail to be deleted, in which case it may contain all or a portion of the files that were being imported. Accessing this folder may be one possible way to retrieve photos/videos that failed to import correctly.
Open Windows File Explorer and set it to show hidden files and folders.
Windows 10: In Explorer click the View tab > click Options > click "Change folders and search options"
Click the View tab
Under "Advanced settings" find "Hidden files and folders" and select "Show hidden files, folders, and drives" > click OK
In Explorer, find the ".Picasa3Temp" folder which could be on the C drive under "My Pictures" / "Pictures" or in your default downloads folder
Right-click on the ".Picasa3Temp" folder and go to Properties, then in the General tab remove the check mark from "Hidden" and click Apply or OK
Rename the folder: right-click (or F2) on the folder and rename it to "CrashedImport" (or any other name) and be sure to remove the period at the front
Start Picasa (if it isn't already running), and go to Tools > Folder Manager
In the tree view on the left side of Folder Manager click Pictures (or the folder where you store photos), then click on the little triangle to the left to expand the subfolders.
Find the subfolder you renamed in step 2 above, and on the right panel, set that folder to "Scan Always"
Click OK to close the Folder Manager, and wait for Picasa to finish scanning the new folder ("CrashedImport")
Go to the folder ("CrashedImport") in Picasa's Folders collection and look at all the photos. Drag any photos you want to keep into other folders.
You can also access the photos in Explorer
Once you have all the photos you want to keep, you can right-click on the folder name ("CrashedImport") and delete that folder (the folder will be deleted from your computer), or ignore it by clicking "Remove from Picasa" (the folder will remain on your computer but will not be visible in Picasa)
If Import failed more times while the ".Picasa3Temp" folder was present, Picasa will create another temporary folder ".Picasa3Temp_1" and so on, so repeat the above directions until you have found all the temporary folders and have retrieved photos/videos from all of them.
If you were not able to recover some or all of the photos/videos from the temporary folders as explained above, do not take pictures using the memory card that had the failed import!
Taking more pictures lessons your chances of recovering photos. Follow this topic: "Recover deleted photos using a file recovery program" to recover the deleted files from the Memory Card.