a) are you sure it is yahoo.com and not yahoo.co.uk or whatever?
b) are you sure it is devallionsmith and not devallion.smith or whatever?
c) are you sure you are using your correct password?
Yahoo have a good comprehensive help facility, both off and online.
To repeat Pip22's comment, you have provided scant detail of what exactly is happening.
Please advise back on progress.
Good Luck - Keep us posted.