I'm sorry to hear about the trouble! As a first step for this, we can try clearing the library cache. To do this, please navigate to the following path (you can just copy & paste this into a File Explorer window):
Delete the contents of the cache3 folder and then try relaunching Axure RP. If the cache3 folder is empty to begin with and remains empty after restarting Axure RP, then it could be that something on your machine is clearing out the library cache automatically. Possible culprits are antivirus/malware, scanners, and the like, which would best be resolved by adding the above path to any relevant whitelists (you may want to reach out to your IT team to assist with this).
If clearing the cache doesn't help, let's check that the Default libraries are in their source folder and that you're able to access them. To check, please browse to the following path:
C:\Program Files (x86)\Axure\Axure RP 8\DefaultSettings\Libraries
The Default, Flow, and Icons library should appear in this folder. Please let me know if you see anything else, and please also confirm that your user account has the necessary permissions to access this folder.
If the three libraries are indeed in this folder and you're able to access them, please open up Axure RP and click on the hamburger button in the Libraries pane to select "Load Library". Navigate to the path of where the Default libraries are stored (the above path) and try manually re-loading them. Does that help?
Let me know what you find!