Nobody can possibly help you without all the details...

Exactly which All-In-One model is this?

Have you checked that power cord is correctly connected to rear of PC?

Have you checked that power cord is connected to a working outlet?

If you're using a power strip or surge protector, have you removed them and connected PC directly to a working outlet?

What color is the power button when it won't boot?

Do you see anything on the monitor?

Exactly what does "won't let me get to the log in screen" mean?

What version of Windows and did you get any software updates yesterday, either via Windows Update or Dell Support Assist?

Etc, etc.

Hard drive has failed and there are no backups.  You are toast. its not a matter of IF they fail its a matter of when.

I found the exact right steps to take and it's working