everytime i try to launch minecraft it always keeps crashing (and this is NOT the first time this has happened!!!)

when i am launching minecraft it keeps crashing (NOT the first time its happened!!!)
What I expected to happen was...:
for minecraft to launch
What actually happened was...:
IT CRASHED.....AGAIN!!!

Steps to Reproduce:
1.....not sure
2......not sure
3. ....not sure just plkease fix all the crashing