St. Thérèse of Lisieux (1873–1897) is famous for promising to send down a shower of roses as signs of her prayers for people after her death. The image of a shower of roses is just that: an image of the graces she promised to ask for and, through his love, obtain from God for those who prayed for her intercession. Some people who have prayed for Thérèse’s intercession have discovered roses in unexpected or unlikely places while praying the novena and have taken such sightings as a tangible sign of Thérèse’s intercession on their behalf.
While such a favor is a lovely reminder of this saint’s intercession, there is something of which you should be careful. If you approach this novena from the perspective that, if you pray it in exactly the right way, you will be rewarded with a rose in answer to your question, you are engaging in superstition.
The Catechism warns,
Superstition is the deviation of religious feeling and of the practices this feeling imposes. It can even affect the worship we offer the true God, e.g., when one attributes an importance in some way magical to certain practices otherwise lawful or necessary. To attribute the efficacy of prayers or of sacramental signs to their mere external performance, apart from the interior dispositions that they demand, is to fall into superstition. (2111)
A novena is simply the pious practice of praying for a particular intention nine times. It is usually done with one prayer over a nine-day period but can be done by praying the same prayer nine times in a row. Any formal, informal, or spontaneous prayer done in a set of nine times constitutes a novena. You may use a formal novena that is approved for devotional use or you may put together your own set of prayers for your private use. The only requirement is that the prayers not be said with the intention to manipulate an answer from God. Whatever they are—if they are said faithfully and with trust in God that he will answer your prayer in the manner he wills to do so—they are fine.
So, in short, no, you don’t have to start over this novena; but you must abandon any thought that saying a novena will be automatically rewarded with the response you have specified.