OsCommerce online store PHP problems

I am experiencing some PHP problems with my oscommerce store, as it says "product not found!" and I am having issues with the logging system / orders. Another problem is that it does not allow to create new users and properly manage the customers, welcome emails, etc. My store used to work properly until my ISP changed servers and it has not worked since, and I get the above error on almost all product views, and the ISP is been unresponsive. I'm looking for an experienced PHP programmer, honest, and that can give me feedback on what to improve on my site, as well as to fix the above problems. It is a rather small store, only 20 standard products, translated in Spanish and English so it is not a huge database of products / sizes. Thank you, Alexis.