It depends what you mean by "lost coins". Usually people confuse coins that haven't moved for a long time with lost coins. If it is the former, they are not lost and what happens is that they eventually move! If it is the later then Satoshi already answered your question:
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.
Feel free to read the rest of that topic as well.
Basically coins that are lost are:
1. Coins sent to a provably unspendable output
This includes the OP_RETURN outputs, broken outputs (eg. OP_HASH160 <push 10 bytes> OP_EQUALVERIFY OP_CHECKSIG) and burn addresses. These are lost forever and it is impossible to spend them.
2. Coins in a wallet/address with lost key
This could be a paper wallet that was lost, a digital wallet (eg. wallet.dat file) that was lost due to hardware damage, encryption password that was forgotten, etc. These are lost forever unless the user remembers their password, finds their paper wallet, etc.