Unable to Log in to MacBookAir account

Sharon, I just went through this myself. I thought I got some virus something. I was close to formatting my drive but found a fix. This looks like an obscure but SERIOUS bug in the software.

I used the instructions on the page below and reset my password under the section "Use the Reset Password assistant" using the Terminal app. I was able to log into my MacOS account with my *current* password and then I used the instructions using the "resetpassword" command to change the password. Then I was able to login. Saved myself; I too am running a business and hadn't backed up my files in the last week.

If you can't reset your Mac login password - Apple Support

Best of luck!

If you forgot your Mac login password - Apple Support has lots and lots of "ifs" due to the various ways Apple implemented device security across many Mac models and many macOS versions.

You already exhausted the first procedure, which is to enter your password "up to three times," but then we are supposed to proceed to the other options. Methodically follow each procedure all the way to the end, and then proceed as under If you can't reset your Mac login password - Apple Support.

Which is probably going to lead to...

I do have an external hard drive which is backed up every day, but have read I can only use this on the account it is saved from.

I don't know what you read or where you read it, but it doesn't seem right to me. The Internet is a vast source of information that just isn't so. Of course you can restore a Mac from its Time Machine backup; that's what it's designed for.

Exhaust all the other steps first.