Sudoku Solver timelimit exceeded (TLE)

Code of sudoku solver for 99 grid here is giving timeout error on coding blocks ide and on my system too, it seems to take forever.
For 4
4, works fine.
My test case:
9 0 0 0 0 0 0 0 8
6 0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0 6
0 1 0 0 0 0 2 8 0
0 0 2 0 1 8 0 0 0
0 0 0 3 6 0 0 0 7
8 0 0 0 0 0 0 0 0
2 0 0 6 0 0 8 0 0
0 0 0 0 0 0 0 2 0

Any way of optimizing?