When you loses wallet, it has the effect of removing money out of circulation. Lost bitcoins still remain in the block chain just like any other bitcoins. However, lost bitcoins remain dormant forever because there is no way for anybody to find the private key(s) that would allow them to be spent again. Because of the law of supply and demand, when fewer bitcoins are available, the ones that are left will be in higher demand and increase in value to compensate.
Sadly, there is no way to retrieve lost Bitcoin if you don't know its private keys. I've heard that there is a person who lost his more than 300 Bitcoin way back in 2010 because he accidentally throwed his laptop in the garbage truck and his Bitcoin were stored in an offline wallet. Those lost Bitcoin will definitely increase its value as the demand increases.