If you have two block-chains of same length and you add your block to one chain while the network selects the other chain, what happens to the transactions in your block ?
That block and the transactions in that block will not be accepted as confirmed by the network.
Few things can happen with these transactions though:
some or all of these transactions are possibly included in the other chain
the transactions that are not included in other chain are returned to the pool of unconfirmed transactions, waiting to be included in some future block
if conflicting transaction is included in other chain than the transaction will be considered invalid (as a double spend) and will be removed from the pool and will not be included in the future