Wasting of calculated hashes
your work is "wasted" caused by the mining-algorithm.

you have found a new block if sha2(sha2(blockheaderOfnewBlock) starts with a certain amount of zeros. the blockheader contains a nonce, the hash of the previous block and some other information. since the previous-block-hash is another hash if a new block was found you have another information-base for your mining-software.