If a block chain is considered invalid, how about transactions in this block chain?
Any transactions in a block that is no longer part of the longest chain will revert to unconfirmed (assuming they're not in the new longest chain) and return to the mempool. In most cases, assuming the transactions remain valid (i.e. the outputs they spend aren't already spent in the new longest chain), said transactions will simply be mined into a new block on the longest chain.