where they where working before and stopped, i would suspect the server has had file upload disabled recently for some reason.
double check the upload settings in the php.ini to ensure that upload is still enabled.
it may also be a permissions error with the folder that you are uploading to, basically something server side has changed to cause the upload to fail.
on a page that you have upload on, add the following code to turn on error reporting:
that may help find the problem if it is a permissions error.