There are actually two types of "lost coins".

If someone loses the private key associated with a bitcoin address, any bitcoins ever received at that address will remain unspendable (assuming that no weaknesses are ever discovered in ECDSA, SHA-256, and RIPEMD-160 in the future).  However, if someone finds that private key (maybe it was written on a piece of paper that was dropped, but then later found by someone else?) all bitcoins associated with that address immediately become spendable again.  In most cases of "lost" bitcoins, the private key was ONLY stored in a file on a computer, and that file is deleted, or destroyed, making it essentially impossible for anyone to ever "find" that private key in the future.

The other type of "lost" coin is very rare and can be identified on the blockchain.  These coins are permanently lost, and there is no way to recover them.  They cannot be recovered if ECDSA, SHA-256, and/or RIPEMD-160 are broken in the future.  They cannot be recovered under any circumstances.  This type of "lost" coin involves someone paying a transaction fee on their transaction, and a miner having a bug in their software that causes them not to claim that transaction fee in their reward.  The fee is paid, but since nobody claimed it, it simply ceases to exist.