Site visitors 's decreased in reports



i installed piwik for tracking my site visitors six months ago. from two months ago my site visitors report decreased suddenly.
i’m sure site visitors does not decreased and i checks my webserver access log and it shows about 8000 hits daily but from a speciefic day my visitors in piwik report decreased from about 10000 daily to about 900. i didnt change anything in piwik server or on my web server.
can anybody help me?