The Internet Identity takes advantage of the Web Authentication (WebAuthn) API to provide secure cryptographic authentication. While WebAuthn is supported by several of the most popular browsers, it is not supported by all. If you have any issues, we recommend you do the following:
To see a walkthrough of Internet Identity watch this demo: https://www.youtube.com/watch?v=oxEr8UzGeBo
If you have your Internet Identity tied to only one device and you lose that one device, you would be locked out. Hence as a best practice we always recommend you to add multiple devices to your Internet Identity.
To add more devices under your existing Internet Identity, please see the guide here.
This may be a simple issue the DFINITY Foundation can help with. Please let us know by filing a ticket here: https://support.dfinity.org/hc/en-us/requests/new
Since the Internet Computer is a general computing environment, you can still have more traditional login systems (username/password) for your app. Several of our community developers are building different authentication models for the Internet Computer not using WebAuthn.