Strangely, you can access QuickBooks Online (QBO) and yet you received an error message. I'll help you with logging into your account.
Let's go over some troubleshooting steps to figure out what's causing this problem. This might be a browser-related issue or a problem with your QuickBooks Online subscription.
To isolate this, please log in using this link to check if you still have an active subscription since you've mentioned that you haven't logged in for several months. Once logged in, go to Products & billing. Then, click on your product to see if it's still active. If so, we can proceed with using a new incognito window and logging into your QBO account using this link: QBO.intuit.com.
Here are the shortcut keys to open a new incognito window:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
If this works, you can go back to your normal browser and clear the cache to remove unwanted site data. Alternatively, you can use a different supported browser.