if user cancels the transaction, then SQL Server will roll the transaction back. A ROLLBACK statement backs out all modifications made in the transaction by returning the data to the state it was in at the start of the transaction. A ROLLBACK also frees resources held by the transaction.

Don't simply restart SQL Server, Sometime, SQL Server has to take much longer time to recovery due to the rollback and You might in the situation none of your users are able to access SQL Server untill the recovery completes.