My suggestion would be to read How to help us help you because sadly, as it is, you’ve not given the community any clues about your HASS installation, and what might have changed to cause an issue.
Here’s a few suggestions:
- Create a full backup, and download it to a separate device
Your HASS storage device could be corrupted, meaning CSS/ HTML/ other files could now be missing even from a backup (e.g. something odd with HACS, custom cards, bespoke CSS - enough to break the GUI?). - Try multiple client devices - including phones, tablets, and other browsers - in case your favourite client software has updated and started blocking something important (e.g. NoScript, ABP, etc).
- Try the HASS app on a phone.
- If you have tried an uninstall (from what hardware? which version? what OS?), does the HASS install work BEFORE you restore from backup?
You are likely feeling very frustrated and annoyed, but sadly we can’t see the world through your eyes unless you share more information.