You've just discovered that your Gmail password is invalid. You probably question if you were hacked or if you simply forgot the password during the subsequent panicky period.
The good news is that you probably weren't hacked; unless you've been logged out for a time, that is unlikely because Google sends out notices and warnings whenever someone checks in on a new device or changes your password.
However, being unable to access your email is annoying, so you probably want to find a solution as soon as you can. Luckily, if you set up any way of recovery at the beginning — a backup email or phone number — it's incredibly simple to reset your Gmail password from your computer or your phone.
Here's how to change your Gmail password on both a computer and a phone.
How to change your Gmail password and how to do so quickly