Sign out of Yahoo and clear your cache.
Since Yahoo! needs to access your cache to keep you logged in and to communicate with your computer when downloading attachments, a full cache can prevent Yahoo Mail from opening attachments correctly.
Go to your browser's "Tools" or "Internet Options" menu and then select the "Delete Temporary Internet files" command to delete your cache.
Reload the page.
lf you have had Yahoo Mail open for a long time, the page may have gone idle, and you may not be able to access your attachment.
Press the "Reload" (F5) button and then log back in to your account.
Ensure that the attachment is not corrupt.
If the sender did not finish uploading before they hit "Send" or there was an error during the upload, the file may be corrupted.
Ask the sender to re-attach the file and send it again.
Check the file format.
Some formats are proprietary and will not open unless specific programs are installed on your computer.
For instance, if you are sent an AAC file, you will not be able to open it unless iTunes is installed on your computer.
if this fails to fix the problem
If you accidentally delete an important email, you may want to reset your Yahoo! email account to a previous time to recover it.
This is possible by filing a Yahoo! email restoration request.
Unfortunately, Yahoo! is unlikely to be able to reset an email account to a time period more than 24 hours in the past, although it may be possible in some cases.
Visit the Yahoo! email recovery site (see Below).
Click on "Please use this form" next to the name of the Yahoo! Mail version you use (All New Yahoo! Mail or Classic).
Select the time period you want to reset your Yahoo! email account to in hours.
For example, if you wanted to go back eight hours, select "8."
Type your Yahoo! username, name and email address.
If you aren't signed in, you'll need to enter your password as well.
Click on "Submit" to submit your request to Yahoo! technical support.