please help me! file wont open!

Dear Everyone,
we are professional musicians and we have to Play the premiere of a guitar trio I arranged in a really big concert in just 2 weeks...
I have to compose the last 4 bars, then the piece is finished.
But now the file wont open anymore and the backup file(the one with , at the end) only has the first 4 pages ... Not the entire piece... And I am 500% sure I saved it!
This is a total disaster...
Please, who can help me!!! I will be forever grateful and even sent you CDs of us... we are really in a shit Situation if suddenly this file doesnt open.... and yesterday it worked perfectly...
PLEASE HELP!!!