Formal:
* Fresh start
* Rebirth
* Renaissance
* Commencement
* Inauguration
* Genesis
* Dawn
* Resurgence
Informal:
* Do-over
* Clean slate
* Second chance
* New chapter
* Turning point
* Fresh start (again, but less formal)
* New leaf
* New dawn
Figurative:
* Blank canvas
* Open door
* Sunrise
* Springtime
The best word to use will depend on the specific context and the tone you want to convey.