Your Microsoft account can become locked if there's a security issue or you enter an incorrect password too many times. Don't fret though, as recovery is a simple procedure which should take only a minute to complete.
First, login to your Microsoft account at account.microsoft.com. You will be informed that your account is locked, which is to be expected at this stage.
Use the form on the page to enter a phone number. This must be capable of receiving SMS messages. Microsoft will send a unique security code to the number.
Once you've got the code, enter it into the form on the webpage to unlock your account. You'll now need to change your password. It can't be the same as your previous password. This is required to ensure third-party actors are locked out of your account, if it was suspicious activity that caused the lock to be enforced.
Once you've reset your password, you should get back into your account. Remember to enter the new password on all your devices – this includes Windows 10 PCs and any apps which use your Microsoft account, such as Outlook and Skype.