Hi Sara, hope you're doing well! Yes, you can fix it and no you don't need to start again which is always a good thing!
A couple of things to try if your ganache isn't working properly are as follows: -
If the ganache is breaking or separating then whisk in a tablespoon of warm milk or cream, keep adding a little at a time until it comes together either that or put it in the fridge and allow the cocoa butter in the chocolate to set.
If the ganache seems grainy just re-heat it and start again.
If it is too thick, then place in a bowl over a simmering saucepan and mix together slowly or place in the microwave to melt. If the ganache becomes too runny, let it cool and set a little at room temperature and then put in the fridge to thicken.
Remove from the fridge allow to bring back to room temp and then whisk; this can then be piped.
Also, check your ratios of chocolate to cream if your fat to liquid quantity isn't right this may cause the mixture to split.