Where are the visitors? Page-Rank Zero After Five Months

[orb123]

We have a well optimized site with several relevant sites linking to us with fairly high PR.

Our traffic is about 130 visitors/day (it was 400 for a while). Our number indexed pages in Google varies from 1000 to 14000. We use Google sitemap with 80000 url:s in it.
Recently we fell down to 20 visitors/day with still 14000 pages in Google.

I assume this has to do with are low PR? It has been zero since the beginning 5 months ago.

Anyone has a solution?

thanks in advance