Upgraded to Monterey but can't sign in. Any suggestions?

This will depend on the specific Mac model you have. While logged into the Guest account, click the Apple menu and About this Mac and reply with your specific Mac model and year.

Apple changed from Intel to Intel w/T2 Security Chip to Apple Silicon (M1 / M1 Pro / M1 Max) chips and the process will vary depending on the hardware and the dependent security changes.

Yes, that's enough information. You have an Intel processor without the T2 Security Chip so that simplifies things.

First thing to do would be to login as Guest, go to Apple menu -> System Preferences -> Users & Groups and unlock the dialog box with the gold lock at the bottom left. Enter your username of your primary account and password. If it unlocks, attempt to create a new user account and check the box to allow the new user to Administer the Mac. Then reboot and see if you can login with that new account.

If this doesn't work, we can try other methods.

Your data in your original account is still there it is just you are now logged on as an administrator in a different account. The trick will be getting to your data as each macOS user is restricted from fully accessing other user home folders.

Do you have an external hard drive other than a Time Machine drive? What I propose is that we recover your data to an external drive and then you can manually copy stuff over. Alternatively, I can show you how to use Terminal move the data to the /Users/Shared folder and grant ownership to the new admin account. Then you can at least copy stuff over that way.

Open Terminal:

Replace <text> with the actual user names.

The sudo command stands for "Super User Do" and you will be prompted for your new admin password.

List the users home folders

ls /Users

Move the <original_account> from /Users/<original_account> to /Users/Shared/<original_account>

sudo mv /Users/<original_account> /Users/Shared

Change Ownership to the new admin account

sudo chown -R <new_admin>:staff /Users/Shared/<original_account>

Then in Finder press Shift+CMD+G and go to /Users/Shared and you should be able to click on the <original_account> and access the files on the Desktop, Documents, etc. You can copy things over to the new account.

You logged on with the same iCloud account, Apple ID? If so the local account on the Mac shouldn't matter you should be able to access the data on iCloud.

When you go to System Preferences -> Apple ID -> Overview if you are not signed into the same Apple ID, go ahead and Sign Out and back in with the correct Apple ID.