When you open up Eagle, but it's telling you "Oops! Library missed", like the image shown below:

The reason being for this is that Eagle can't find the library directory; To solve this, first, go to the directory path where your library is stored and check what went wrong. The path can be found in the notification, please see below:

Common causes of the issue, and solutions to them are listed below, please see:

The path of the library has been changed.
You may have accidentally moved or renamed the library. Please search for the name or the keyword ".library" through the device to locate the new directory of the library, and relocate it within Eagle to restore.
Deleted library by mistake.
If you can't find any result that is related to ".library", it could be that the library has somehow been deleted. This often occurs when the library was stored at the desktop, if this is the case, please try to restore the folder from the recycle bin.
Using a third-party sync tool, but the sync process didn't complete.
If you are using the sync tool (e.g. iCloud, OneDrive, Dropbox) and you are sure that the library path is correct, this could be that Eagle's essential data files was not completely synced yet. Please make sure that the syncing progress is completed.
Exception in the portable hard drive or the transferring process.
If I/O error, Unknown error was shown when you try to load the application, it could mean that the error is related to the hard drive, for example, Incompatible formats or bad sectors. Please check the hard disk compatibility to the OS, and try to check/repair with the Disk optimizer/repair tools.

If you want to search for files on your device, we recommend the app "Everything" for Windows, and the "Spotlight" for macOS.

If you have found the library location or restored the library that has been deleted accidentally, you can reopen it in the following ways:

Click the "Open Existing Library" button on the right
Select the library path, click the open button
After the library is successfully loaded, the process is complete!

If these didn't help, please contact us and provide the software log and your results after the above-mentioned troubleshooting, we'll try our best to help you out.

I can't open Library, it prompted message "Oops! Library missed" after I open it