My Vendor List has completely disappeared. But when I try to add them back, it says they already exist. I have updated the app. I can't seem to log on the website either.

Welcome to the Community, savannahhuey.

Since you're unable to see your Vendors list in your QuickBooks desktop and/or mobile apps, I'd recommend uninstalling, then reinstalling them.

If the issue continues, you can try clearing or offloading app data for mobile apps.

Here's how to clear data for Android devices:
 

1. Go to your device settings.
2. Tap Apps.
3. Choose QuickBooks Online.
4. Go to the Storage section.
5. Select Clear Data.

Here's how to offload apps on iOS devices:
 

1. Go to your device settings.
2. Access the General screen and hit iPhone Storage.
3. Tap QuickBooks Online.
4. Select Offload App.

These processes will free up storage space without deleting documents or data from your apps.

In regard to troubles signing in with a web browser, I'd recommend recovering the Intuit Account if you can't sign in.

Please don't hesitate to send a reply if there's any additional questions. Have an awesome Thursday!