On the back burner

If you put something 'on the back burner' (which is the back part of the stove), it means that now you have to attend to other things first before you can get back to it.

Example:
-I was planning to replace my kitchen floor this summer, but now we have to get a new car.
-Well, I guess the floor will have to go on the back burner.

Back to Square One

'Back to Square One' is where you find yourself when you have to start something that you have worked on all over again, from the beginning.

Example:
-Well, since our supervisor didn't like our ideas, we're back to Square One.
-Ugh.